Power transmission device and wireless power transmission system

ABSTRACT

A power transmission device includes an inverter, an oscillator, a foreign substance detector, and a power transmission control circuitry. The power transmission control circuitry causes the foreign substance detector to perform a series of multiple processes and determine whether a foreign substance is present before a transmission of first AC power starts, and then causes the inverter to start the transmission of the first AC power. After the transmission starts, a detection period in which foreign substance detecting is performed and a power transmission period in which transmission of the first AC power is performed are repeated. The series of multiple processes is divided and performed in the multiple repeated detecting periods. The foreign substance detector is caused to divide and perform the series of multiple processes using the detecting periods and determine whether a foreign substance is present.

BACKGROUND

1. Technical Field

The present disclosure relates to a power transmission device forwirelessly transmitting electric power and a wireless power transmissionsystem.

2. Description of the Related Art

In recent years, electronic devices and EV equipment, such as mobilephones and electric vehicles, which involve mobility have been inwidespread use. Development of a wireless power transmission system forsuch equipment has been under way. Wireless power transmissiontechniques include an electromagnetic induction method, a magnetic fieldresonance method (resonant magnetic field coupling method), an electricfield coupling method, and the like.

A wireless power transmission system of either of the electromagneticinduction method and the magnetic field resonance method includes apower transmission device with a power transmission coil and a powerreception device with a power reception coil. The power transmissiondevice is enabled to transmit power to the power reception device,without requiring direct contact of their electrodes, in such a way thatthe power reception coil captures a magnetic field generated by thepower transmission coil. The wireless power transmission system of themagnetic field resonance method is disclosed in Japanese UnexaminedPatent Application Publication No. 2009-33782 (hereinafter, referred toas JP2009-33782A), for example.

SUMMARY

In such conventional techniques, however, there is a need for a powertransmission device of a wireless power transmission system capable ofdetecting a foreign substance with high accuracy even after powertransmission starts.

In one general aspect, the techniques disclosed here feature a powertransmission device comprising:

-   -   an inverter that generates first AC power and transmits the        first AC power wirelessly to a first resonator of a power        receiving device via a second resonator;    -   an oscillator that generates second AC power which is smaller        than the first AC, and transmits the second AC power to the        first resonator via a third resonator;    -   a foreign substance detector that performs a series of multiple        processes thereby to determine whether or not a foreign        substance is present between the first resonator and the third        resonator based on a physical quantity at the third resonator,        the physical quantity varying depending on the second AC power;        and    -   power transmission control circuitry operative to:    -   cause the foreign substance detector to perform the series of        multiple processes before a start of a transmission of the first        AC power;    -   cause the inverter to start the transmission of the first AC        power if the foreign substance detector determines that the        foreign substance is not present;    -   repeat a foreign substance detection period and a power        transmission period alternately where the foreign substance        detection period is a period in which the foreign substance        detector performs one of the series of multiple processes and        the power transmission period is a period in which the inverter        transmits the first AC power, after the start of the        transmission of the first AC power; and    -   cause the foreign substance detector to divide the series of        multiple processes and determine whether or not the foreign        substance is present as a result of performing all of the        divided series of multiple processes.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

According to one aspect of the present disclosure, there can be provideda power transmission device of a wireless power transmission systemcapable of implementing foreign substance sensing with high accuracyeven after power transmission starts.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an overview of operation in a wirelesspower transmission system;

FIG. 2 is a diagram illustrating a delay period and operation of a powerreception device based on the delay period;

FIG. 3 is a flowchart illustrating an example of operation in the powerreception device;

FIG. 4 is a diagram illustrating an example of a series of multipleprocesses divided in the present disclosure;

FIG. 5 is a diagram illustrating other aspects of a foreign substancesensing operation of the present disclosure;

FIG. 6 is a diagram illustrating an example of effect of shortening apower transmission stop period in an embodiment of the presentdisclosure;

FIG. 7 is a diagram illustrating a schematic configuration of thewireless power transmission system in an embodiment 1 of the presentdisclosure;

FIG. 8 is a diagram illustrating a more detailed configuration of apower transmission circuit in the embodiment 1;

FIG. 9 is a diagram illustrating a configuration example of anoscillation circuit;

FIG. 10 is a diagram illustrating timing of charging and foreignsubstance sensing;

FIG. 11 is a diagram illustrating an operating principle of a couplingcoefficient estimation method used in foreign substance sensing;

FIG. 12 is a diagram illustrating a specific circuit configurationexample of a power transmission device and a power reception device;

FIG. 13 is a flowchart illustrating a flow of a foreign substancesensing process based on a coupling coefficient;

FIG. 14 is a flowchart illustrating other example of the foreignsubstance sensing process;

FIG. 15 is a diagram illustrating a first example of a method forsetting a threshold;

FIG. 16 is a diagram illustrating a second example of the method forsetting a threshold;

FIG. 17 is a diagram illustrating a third example of the method forsetting a threshold;

FIG. 18 is a diagram illustrating a fourth example of the method forsetting a threshold;

FIG. 19 is a diagram illustrating a first example of process division inthe embodiment 1;

FIG. 20A is a diagram showing results of measurements of a couplingcoefficient and an input inductance value in each of a case in which analuminum foreign substance is present and a case in which an aluminumforeign substance is not present, for seven models of evaluationterminals;

FIG. 20B is a diagram showing results of measurements of a couplingcoefficient and a coil-end voltage in each of a case in which an ironforeign substance is present and a case in which an iron foreignsubstance is not present, for the seven models of evaluation terminals;

FIG. 21 is a diagram illustrating a second example of the processdivision in the embodiment 1;

FIG. 22 is a diagram illustrating a third example of the processdivision in the embodiment 1;

FIG. 23 is a diagram illustrating a fourth example of the processdivision in the embodiment 1;

FIG. 24 is a diagram illustrating a fifth example of the processdivision in the embodiment 1;

FIG. 25 is a diagram illustrating a variation of the fifth example ofthe process division in the embodiment 1;

FIG. 26 is a diagram illustrating a sixth example of the processdivision in the embodiment 1;

FIG. 27 is a diagram illustrating a configuration of a wireless powertransmission system in an embodiment 2;

FIG. 28 is a diagram illustrating a detailed configuration of a powertransmission circuit in the embodiment 2;

FIG. 29 is a diagram illustrating a configuration of a selector switchin the embodiment 2;

FIG. 30 is a first diagram showing a sensing result that determinespresence or absence of a foreign substance, using the seven models ofevaluation terminals;

FIG. 31 is a second diagram showing a sensing result that determinespresence or absence of a foreign substance, using the seven models ofevaluation terminals;

FIG. 32 is a third diagram showing a sensing result that determinespresence or absence of a foreign substance, using the seven models ofevaluation terminals;

DETAILED DESCRIPTION

(Findings as Basis for the Present Disclosure)

The inventors found that the power transmission device in the wirelesspower transmission system described in “BACKGROUND” have the followingproblems.

First, a definition of a “foreign substance” is described. In thepresent disclosure, a “foreign substance” means an object, such as metalor a human body (animal), which generates heat due to electric powertransmitted between a power transmission coil (or a coil for sensing aforeign substance) and a power reception coil when the object is locatedadjacent to the power transmission coil or the power reception coil.

Next, operation of a power transmission device is described. First, whena power switch of the power transmission device is turned on, the powertransmission device performs alignment of the power transmission coil ofthe power transmission device and the power reception coil of a powerreception device. The “alignment” means operation of detecting a powertransmission resonator (including the power transmission coil) in thepower transmission device and a power reception resonator (including thepower reception coil) in the power reception device being arranged witha positional relationship suitable for transmission of electric power.When completing the alignment of the power transmission coil and thepower reception coil, the power transmission device performs foreignsubstance sensing to determine whether or not a foreign substance ispresent between the power transmission coil and the power receptioncoil. The foreign substance sensing may be performed by, for example,detection of a change in a physical quantity such as a voltage appliedto the power transmission coil. When determining that no foreignsubstance is present between the power transmission coil and the powerreception coil, the power transmission device transmits AC power fromthe power transmission coil to the power reception coil in a contactlessmanner.

However, even after the determination that no foreign substance ispresent between the power transmission coil and the power receptioncoil, a foreign substance may possibly enter between the coils duringpower transmission. For example, a case is assumed in which the powertransmission device is a charging stand installed within a vehicle andthe power reception device is mounted in an instrument (power receptionterminal) capable of contactless charging, such as a smart phone, atablet terminal, a mobile phone, or the like. In such a case, it islikely that a sway movement of a vehicle body during running may cause aforeign substance such as a coin to enter between the power transmissioncoil and the power reception coil under the charging operation. If aforeign substance enters between the power transmission coil and thepower reception coil as mentioned above, an eddy current may begenerated in the foreign substance and the foreign substance may beoverheated.

In order to prevent overheating of a foreign substance as describedabove, the inventors have been considering a solution in which, afterstarting the power transmission, the power transmission device performsmonitoring to prevent a foreign substance from being overheated, byrepeating a foreign substance sensing session for performing foreignsubstance sensing and a power transmission session for performing powertransmission.

FIG. 1 is a diagram illustrating an overview of operation in a wirelesspower transmission system under consideration of the inventors. In thissystem, a power transmission device first performs foreign substancesensing (detecting) and starts power transmission after determining thatthere is no foreign substance (initial sensing and initial powertransmission). When a certain period of time (a few seconds, forexample) elapses after the power transmission starts, the powertransmission device stops power transmission and performs foreignsubstance sensing again. Thereafter, the power transmission devicerepeats the power transmission and the foreign substance sensing. Suchoperations enable the power transmission device to monitor an entry of aforeign substance while continuing power transmission.

Meanwhile, JP2009-33782A discloses a system that uses one powertransmission coil and one power reception coil, and detects a foreignsubstance based on the waveform of an induced voltage of the powertransmission coil. A power transmission device in this system senses aforeign substance by using a frequency different from a powertransmission frequency before power transmission starts. On the otherhand, during power transmission, the power transmission deviceperiodically senses a foreign substance using the same frequency as thepower transmission frequency, while transmitting electric power.

For foreign substance sensing during power transmission, one foreignsubstance sensing session specifically involves the following processes.That is to say, the power transmission device measures voltage waveformof the same frequency as the power transmission frequency once, computespulse width of the voltage waveform, and determines whether or not aforeign substance is present, based on an amount of change in the pulsewidth from a reference value.

In this way, the power transmission device in JP2009-33782A performs aseries of multiple processes including a measurement process, acomputation process, and a determination process, in one foreignsubstance sensing session.

However, the inventors of the present disclosure found that thefollowing problems occur in the foreign substance sensing methoddisclosed in JP2009-33782A.

The power transmission device in JP2009-33782A performs foreignsubstance sensing using the same frequency as the power transmissionfrequency during power transmission. In general, electric power duringpower transmission is much larger than that during foreign substancesensing. For example, the electric power during power transmission isapproximately 100 to approximately 1000 times larger than that duringforeign substance sensing. Thus, in the system in JP2009-33782A, avariation in voltage amplitude due to the presence of a foreignsubstance is smaller than a variation in voltage amplitude during powertransmission. Consequently, an SN ratio is not sufficiently large, whichmakes it difficult to perform foreign substance sensing with highaccuracy. Furthermore, in the system in JP2009-33782A, since foreignsubstance sensing is performed using the same frequency as the powertransmission frequency during power transmission, the accuracy of theforeign substance sensing may be low due to an influence of the powertransmission.

To address the problems described above, one possible solution is toprovide a foreign substance sensing coil in addition to a powertransmission coil and perform foreign substance sensing at a frequencydifferent from a power transmission frequency during power transmission,in the same manner as that before power transmission starts.

In this possible solution, however, multiple harmonics or the like ofelectric power during power transmission affect the foreign substancesensing coil and causes noise in the foreign substance sensing coil.Therefore, it is still difficult to perform foreign substance sensingwith high accuracy during power transmission even though the foreignsubstance sensing coil is provided.

JP2009-33782A also has a problem that foreign substance sensing withhigh accuracy is difficult to perform because the waveform of a singlephysical quantity (voltage) is measured only once in one time of foreignsubstance sensing. More specifically, although there are various foreignsubstances of various materials or shapes, the foreign substances ofvarious materials or shapes cannot be sensed with the method inJP2009-33782A.

Sensing of such various foreign substances with high accuracy requires aseries of multiple processes including a process to measure one or morephysical quantities (a voltage applied to a power transmission coil, afrequency of the voltage, or the like, for example) multiple times(measurement process), a process to compute an index value (couplingcoefficient or the like, for example) to be used in determination on thepresence or absence of a foreign substance based on the physicalquantities obtained from the multiple times of measurement (computationprocess), and a process to determine whether or not a foreign substanceis present (determination process). A “physical quantity” herein means acoil-related quantity expressed in an electrical unit, such as a voltageapplied to the power transmission coil, a current flowing to the powertransmission coil, a frequency of the voltage applied to the powertransmission coil, an input impedance value of the power transmissioncoil, or an input inductance value of the power transmission coil. Inaddition, in order to avoid an influence of power transmission, foreignsubstance sensing needs to be performed with the power transmissionstopped.

Execution of the series of multiple processes described above, however,entails a problem that a period of one foreign substance sensing sessionis long. It may be possible indeed to perform foreign substance sensingwith high accuracy by setting a long period for one foreign substancesensing session and performing many processes in each session. It is notpreferable, however, to stop power transmission for a long period oftime by allocating a long period to each foreign substance sensingsession. When a proportion of length of a foreign substance sensingsession to length of a power transmission session is large, theefficiency of power transmission is low. For example, when the powertransmission device is a wireless charger, it takes time to completecharging a load (secondary battery, for example) of a power receptiondevice after power transmission starts.

As described above, the inventors found that an effective method inorder to sense a foreign substance with higher accuracy than the foreignsubstance sensing method disclosed in JP2009-33782A is to perform aseries of multiple processes and to divide a foreign substance sensingsession and a power transmission session. The inventors found a problem,however, that the above method results in lowering of the powertransmission efficiency due to an increase in a proportion of time(power transmission stop time) in which foreign substance sensing isperformed to power transmission time in which power transmission isperformed.

When a power reception device is a smart phone, in particular, eachmanufacturer sets length of a period (referred to as a delay period)from when power transmission is stopped to when a notification unit(lamp, for example) of the power reception device notifies the stop ofpower transmission. Length of a delay period varies depending on amanufacturer and a model. The length may be set to length fromapproximately 5 msec to 10 msec, for example.

FIG. 2 is a diagram illustrating a delay period set in a power receptiondevice and operation of the power reception device based thereon. FIG.2(a) illustrates length TA1 of a delay period. FIG. 2(b) illustrates anexample in which power transmission stop time T1 is longer than thelength TA1 of the delay period. FIG. 2(c) illustrates a case in whichpower transmission stop time TS1 is shorter than the length TA1 of thedelay period.

As illustrated in FIG. 2(b), when the time T1 (power transmission stoptime) in which power transmission (more specifically, charging) isstopped exceeds the length TA1 of delay period, a power reception moduleof a smart phone turns off a lamp (lamp indicating that charging isongoing) which is lighted in the smart phone. When foreign substancesensing ends and power transmission is resumed after the stop of powertransmission exceeds the length TA1 of delay period, the power receptionmodule turns on the lamp again indicating that charging is ongoing.

On the other hand, as illustrated in FIG. 2(c), when the powertransmission stop time TS1 is equal or less than the length TA1 of delayperiod, the power reception module keeps the lamp in a lighted state.When power transmission is resumed after the power transmission stopperiod TS1 elapses, the power reception module resumes power receptionwith the lamp remaining lighted.

FIG. 3 is a flowchart illustrating the above operation in the powerreception device. In step S301, when the power reception device sensesreception of electric power, it proceeds to step S302 where the powerreception device turns on a charging indicator (the lamp describedabove, for example). Then, in step S303, when the power reception devicesenses stop of power reception, it proceeds to step S304 where the powerreception device determines for every certain time whether it sensespower reception. Here, if Yes is determined, the power reception deviceproceeds to step S305 where it resumes power reception. If No isdetermined, the power reception device proceeds to step S306 where itdetermines whether a delay time has elapsed after the power receptiondevice senses stop of power reception. Here, if the power receptiondevice determines on Yes, it proceeds to step S307 where the powerreception device turns off the charging indicator. If the powerreception device determines on No, it returns to step S304 where thepower reception device determines on sensing of power reception again.

In addition, it may not necessarily be the power reception device thatdirectly performs the operation of turning on or off the chargingindicator. For example, the power reception device has only to give acommand to turn on or off to a power reception terminal mounted in thepower reception module and the power reception terminal may actuallyturn on or off the charging indicator.

If a charging method of repeating a foreign substance sensing sessionand a power transmission session is applied to the power receptiondevice that performs the operations described above, the chargingindicator repeats flashing when the foreign substance sensing session islong. In an onboard charging system, in particular, if a lamp of a smartphone repeatedly turns on or off (flashes) while a user is driving, theuser may become distracted.

To summarize the above, in order to perform foreign substance sensingwith high accuracy when a foreign substance sensing session in whichforeign substance sensing is performed and a power transmission sessionin which power transmission is performed are repeated after a powertransmission device starts power transmission, it is necessary toperform a series of multiple processes as described above. It was foundout, however, that in order to perform all of the series of multipleprocesses, it was necessary to extend power transmission stop time. Theinventors found a problem that due to this, the power transmissionefficiency was reduced and a user might become distracted by a flashinglamp while he/she was driving.

Therefore, there is desired a power transmission device that canimplement foreign substance sensing with high accuracy even after powertransmission starts, while preventing reduction of the powertransmission efficiency by shortening power transmission stop time whena foreign substance sensing session and a power transmission session arerepeated after the power transmission device starts power transmission.Furthermore, there is desired a power transmission device that shortenspower transmission stop time and keeps lighted an indicator (lamp, forexample) indicating that charging is ongoing.

With the above consideration, the inventors arrived at each aspect to bedisclosed below.

A power transmission device according to one aspect of the presentdisclosure is a power transmission device comprising:

-   -   an inverter that generates first AC power and transmits the        first AC power wirelessly to a first resonator of a power        receiving device via a second resonator;    -   an oscillator that generates second AC power which is smaller        than the first AC, and transmits the second AC power to the        first resonator via a third resonator;    -   a foreign substance detector that performs a series of multiple        processes thereby to determine whether or not a foreign        substance is present between the first resonator and the third        resonator based on a physical quantity at the third resonator,        the physical quantity varying depending on the second AC power;        and    -   power transmission control circuitry operative to:    -   cause the foreign substance detector to perform the series of        multiple processes before a start of a transmission of the first        AC power;    -   cause the inverter to start the transmission of the first AC        power if the foreign substance detector determines that the        foreign substance is not present;    -   repeat a foreign substance detection period and a power        transmission period alternately where the foreign substance        detection period is a period in which the foreign substance        detector performs one of the series of multiple processes and        the power transmission period is a period in which the inverter        transmits the first AC power, after the start of the        transmission of the first AC power; and    -   cause the foreign substance detector to divide the series of        multiple processes and determine whether or not the foreign        substance is present as a result of performing all of the        divided series of multiple processes.        Here, an inverter is also referred to as “an inverter circuit”,        an oscillator is also referred to as “an oscillation circuit”, a        foreign substance detector is also referred to as “a foreign        matter detection circuit”, or power transmission control        circuitry is also referred to as “a control circuit”.

According to the aspect described above, the power transmission controlcircuitry causes the foreign substance sensing judgment circuit (alsoreferred to as “a foreign substance detector”) to perform the series ofmultiple processes to determine whether or not a foreign substance ispresent, before transmission of the first AC power starts, and thencauses the inverter circuit to start transmission of the first AC power.

After the transmission of the first AC power starts, the powertransmission control circuitry repeats a foreign substance sensingsession (detection period) in which the foreign substance sensing isperformed and a power transmission session (detection period) in whichpower transmission of the first AC power is performed.

Repetition of a foreign substance sensing session and a powertransmission session causes multiple foreign substance sensing sessions.In the multiple foreign substance sensing sessions, the dividedprocesses in the series of multiple processes are performed. The powertransmission control circuitry causes the foreign substance sensingjudgment circuit to perform entire processing including all theprocesses divided in the series of multiple processes (total processingincluding the processes) and determine whether or not a foreignsubstance is present.

This can shorten length of one foreign substance sensing session, thusreducing a proportion of time to perform foreign substance sensing topower transmission time to perform power transmission (specifically,shortening of power transmission stop time). Thus, reduction of thepower transmission efficiency can be prevented. This can also shortenlength of one foreign substance sensing session (specifically, shortenthe power transmission stop time). For example, the power transmissionstop time can be made shorter than length of a delay period from whenpower transmission is stopped to when the power transmission stop isnotified by means of a notification unit of a power reception device.Consequently, a lamp indicating that charging is ongoing can becontinuously kept lighted.

Furthermore, foreign substance sensing with high accuracy can beperformed by causing the foreign substance sensing judgment circuit toperform the total processing including the processes divided in theseries of multiple processes and determine whether or not a foreignsubstance is present.

In the following, an overview of power transmission operation andforeign substance sensing operation in the present disclosure aredescribed with reference to FIG. 4 to FIG. 6.

FIG. 4 is a diagram illustrating an example of a series of dividedmultiple processes in the present disclosure. The upper row in FIG. 4illustrates an example of a case in which as with the system inJP2009-33782A, foreign substance sensing based only on one type ofphysical quantity is performed during two consecutive power transmissionsessions. The middle row in FIG. 4 illustrates an example of a case inwhich foreign substance sensing with accuracy improved by the series ofmultiple processes is performed during two consecutive powertransmission sessions. In this example, since the series of multipleprocesses is performed, one foreign substance sensing session is longerthan foreign substance sensing based only on one type of physicalquantity. The lower row in FIG. 4 illustrates an example of a case inwhich one foreign substance sensing session is controlled to be short bydividing and performing the series of multiple processes in repeatedmultiple foreign substance sensing sessions.

In this example, the series of multiple processes is divided to fourprocesses of a first measurement process, a second measurement process,a computation process, and a determination process. The firstmeasurement process may be a process to measure an input inductancevalue Lin(f1) of a power transmission coil when an oscillation circuitoscillates at a first frequency f1 which is lower than a resonancefrequency fr of a power reception resonator, for example. The secondmeasurement process may be a process to measure the input inductancevalue Lin(f1) of the power transmission coil when the oscillationcircuit oscillates at a second frequency f2 which is higher than theresonance frequency fr of the power reception resonator, for example.The computation process may be a process to compute a couplingcoefficient k from two input inductance values L1 and L2, using anexpression of k≈1−Lin(f2)/Lin(f1), for example. The determinationprocess may be a process to determine whether or not the computedcoupling coefficient k has fallen below a predetermined threshold. Withthese four processes, a foreign substance between the power transmissioncoil and the power reception coil can be detected. These four processesare described in detail below.

FIG. 5 is a diagram illustrating other aspects of foreign substancesensing operation of the present disclosure. In this example, of theseries of multiple processes, the power transmission device divides andperforms only the first measurement process and the second measurementprocess in the multiple foreign substance sensing sessions, and performsthe following computation process and determination process in the powertransmission session. In this way, measurement of a physical quantityincluded in the series of multiple processes may be divided andperformed in repeated multiple foreign substance sensing sessions, whilethe rest of the series of multiple processes other than the measurementof the physical quantity may be divided and performed in repeatedmultiple power transmission sessions. More specifically, only some ofthe series of multiple processes may be divided and performed inmultiple foreign substance sensing sessions. According to this aspect,the proportion of the power transmission stop period to the entire powertransmission period can be further reduced.

FIG. 6 is a diagram illustrating an example of effect of shortening apower transmission stop period in an embodiment of the presentdisclosure. In this example, a series of multiple processes includes aprocess to sense a foreign substance made of aluminum (which may behereinafter referred to as an “aluminum foreign substance”) and aprocess to sense a foreign substance made of iron (which may behereinafter referred to as an “iron foreign substance”). As describedbelow, an aluminum foreign substance may be detected based on a changein input inductance of a power transmission coil or a foreign substancesensing coil, for example. As described below, an iron foreign substancemay be detected based on a change in voltage applied to a powertransmission coil or a foreign substance sensing coil, for example.

In the example illustrated in FIG. 6, length TA1 of a delay period is 5msec and a period TS1 taken to take an aluminum foreign substance and aperiod TS2 taken to detect an iron foreign substance are 4 msec. Thus,as illustrated in the middle row of FIG. 6, when sensing of an aluminumforeign substance and sensing of an iron foreign substance are performedconsecutively, length of one foreign substance sensing session isapproximately 8 msec. In this case, since the foreign substance sensingsession is longer than the delay period, the problem of flashingindicator (more specifically, the notification unit) of the powerreception device occurs.

On the other hand, as illustrated in the lower row of FIG. 6, whensensing of an aluminum foreign substance and sensing of an iron foreignsubstance are divided and performed, length of one foreign substancesensing session is approximately 4 msec. In this case, since the foreignsubstance sensing session is shorter than the delay period, the problemof flashing indicator of the power reception device can be avoided.

More specific embodiments of the present disclosure are describedhereinafter with reference to drawings. Note that the present disclosureis not limited to the following embodiments. A new embodiment may beconfigured by making various modifications to each embodiment orcombining multiple embodiments. In the following description, a same orsimilar component is assigned a same reference numeral.

(Embodiment 1)

FIG. 7 is a block diagram illustrating a schematic configuration of awireless power transmission system according to an embodiment 1 of thepresent disclosure. The wireless power transmission system in thisembodiment includes a power transmission circuit 1000, a pair of a powertransmission resonator and a power reception resonator 1010, a detectionresonator 1011, and a power reception circuit 1020. The powertransmission circuit 1000 is configured to convert direct current (DC)energy (electric power) inputted from a direct current power supply 1030into an AC (AC) energy (electric power) and output it. The pair of thepower transmission resonator and the power reception resonator 1010 isconfigured to wirelessly transmit the AC energy outputted from the powertransmission circuit 1000. The pair of the power transmission resonatorand the power reception resonator 1010 consists of the pair of the powertransmission resonator (also referred to as a power transmissionantenna) 1010 a and the power reception resonator (also referred to as apower reception antenna) 1010 b. Each of the power transmissionresonator 1010 a, the detection resonator 1011, and the power receptionresonator 1010 b is configured by a resonance circuit including a coiland a condenser. The pair of the power transmission resonator and thepower reception resonator 1010 wirelessly transmits to the powerreception circuit 1020 the AC energy outputted from the powertransmission circuit 1000 by electromagnetic induction or magnetic fieldresonance. The power reception circuit 1020 converts the AC energytransmitted by the pair of the power transmission resonator and thepower reception resonator 1010 to the direct current energy and suppliesthe direct current energy to a load 1040. The detection resonator 1011is used when a foreign substance is sensed.

In this embodiment, the power reception resonator functions as a firstresonator for receiving first AC power. The power transmission resonatorfunctions as a second resonator for transmitting the first AC power to apower reception device in a contactless manner. The detection resonatorfunctions as a third resonator for transmitting second AC power, whichis smaller than the first AC power, to the power reception device in acontactless manner.

The power transmission circuit 1000 and the power transmission resonator1010 a may be mounted in the power transmission device. The powerreception resonator 1010 b, the power reception circuit 1020, and theload 1040 may be mounted in the power reception device. A powerreception device may be mounted in an electronic device such as a smartphone, a tablet terminal, a mobile terminal or in a motor-driven machinesuch as an electric vehicle. A power transmission device may be acharger that wirelessly supplies electric power to a power receptiondevice. The load 1040 may be equipment having a secondary battery, forexample. The load 1040 may be charged by the direct current energyoutputted from the power reception circuit 1020.

As described below in detail, the power reception resonator 1010 b is aparallel resonance circuit including a power reception coil and acondenser which is connected in parallel to the power reception coil. Aresonance frequency is set to a predetermined value fr. The AC energythat the power reception resonator 1010 b wirelessly receives from thepower transmission resonator 1010 a by way of a space is transmitted tothe power reception circuit 1020.

The power reception circuit 1020 has a rectification circuit 1021connected to the power reception resonator 1010 b and the load 1040, anoutput detection circuit 1022 connected to the rectification circuit1021, and a transmission circuit 1023 connected to the output detectioncircuit 1022. The rectification circuit 1021 converts the AC energytransmitted from the power reception resonator 1010 b into the directcurrent energy and outputs it to the load 1040. The output detectioncircuit 1022 detects at least one of a voltage given to the load 1040 ora current flowing through the load 1040. The transmission circuit 1023conveys to the power transmission circuit 1000 a signal (hereinafterreferred to as a “feedback signal”) indicative of a detection result bythe output detection circuit 1022.

The power transmission circuit 1000 has an inverter circuit 1001, aforeign substance detection circuit 1004, a reception circuit 1005, anda power transmission control circuitry 1090. The inverter circuit 1001is connected to a power supply 1030, converts the direct current energyinputted from the power supply 1030 into the AC energy by multipleswitching elements and outputs it. The foreign substance detectioncircuit 1004 is connected to the detection resonator 1011 and performs aprocess to detect a foreign substance 1050 in the vicinity of thedetection resonator 1011. The reception circuit 1005 receives a feedbacksignal transmitted from the transmission circuit 1023. The controlcircuit 1090 controls each circuit in the power transmission circuit1000 so that a power transmission process using the inverter circuit1001 and a foreign substance sensing process using the foreign substancedetection circuit 1004 are repeated.

Inductance of the coil and capacity of the condenser in the detectionresonator 1011 are adjusted so that the detection resonator 1011resonates at a resonance frequency fr which is same as the powerreception resonator 1010 b.

FIG. 8 is a block diagram illustrating a more detailed configuration ofthe foreign substance detection circuit 1004 and the control circuit1090 in FIG. 7. The foreign substance detection circuit 1004 has anoscillation circuit 1003 and a foreign substance sensing judgmentcircuit 1008. The foreign substance sensing judgment circuit 1008 has ameasurement circuit 1006 and a judgment circuit 1007.

The oscillation circuit 1003 is connected to the power transmissionresonator 1010 a. In a foreign substance sensing session, theoscillation circuit 1003 supplies a voltage including an AC component tothe detection resonator 1011. This couples the detection resonator 1011and the power reception resonator 1010 b electromagnetically. Theoscillation circuit 1003 may be a self-exciting oscillation circuitbased on the LC resonance principle, such as a Colpitts oscillationcircuit, a Hartley oscillator, a clap oscillator, a Franklin oscillatorcircuit, or a pierce oscillator circuit.

FIG. 9 is a diagram illustrating an example of a circuit configurationof the oscillation circuit 1003. The oscillation circuit 1003illustrated in FIG. 9 is a pierce oscillator circuit that functions as aself-exciting LC oscillation circuit. Use of a self-exciting oscillationcircuit enables conversion of a change in input inductance of thedetection resonator 1011 into a change in an oscillatory frequency.Input inductance or a coupling coefficient can be estimated based onsuch an oscillatory frequency of the oscillation circuit 1003. If inputinductance or a coupling coefficient can be estimated, it is possible tojudge presence or absence of a foreign substance inserted in thevicinity of the detection resonator 1011 based on that change. In aconfiguration in which input inductance is directly measured and used,the oscillation circuit 1003 is not necessarily a self-excitingoscillation circuit.

The measurement circuit 1006 measures at least one physical quantity,such as an oscillatory frequency of the oscillation circuit 1003 or anoutput voltage, which varies as a foreign substance approaches thedetection resonator 1011. Physical quantities that change as a foreignsubstance approaches include, for example, input inductance of thedetection resonator 1011, an oscillatory frequency, an output voltage oran output current of the oscillation circuit 1003, a couplingcoefficient of the detection resonator 1011 and the power receptionresonator 1010 b, a Q value, or the like. It can be stated that thesephysical quantities are physical quantities which vary depending oninput impedance of the detection resonator 1011. Therefore, it can bestated that a foreign substance detection process in this embodiment isa process to judge on presence or absence of a foreign substance basedon a change in input impedance of the detection resonator 1011.

The judgment circuit 1007 judges presence or absence of a foreignsubstance based on an amount of change from a reference value, of atleast one physical quantity measured by the measurement circuit 1006. Areference value may be a value of the physical quantity when thedetection resonator 1011 and the power reception resonator 1010 b areelectromagnetically coupled and a foreign substance is sufficiently awayfrom these resonators. When a difference between a measured value andthe reference value exceeds a predetermined threshold, for example, thejudgment circuit 1007 judges that a foreign substance is present.Alternatively, when a value obtained from calculation using multiplemeasured physical quantities falls within a predetermined range, thejudgment circuit 1007 may judge that a foreign substance is present. Thejudgment circuit 1007 stores at least one of a measurement result and ajudgment result in a memory (result storage unit) 1093 in the controlcircuit 1090. The judgment circuit 1007 may call a result of the lastjudgment process from the result storage unit 1093 and combine it with anew detection result to judge whether there is a foreign substance.

At least a part of the measurement circuit 1006 and at least a part ofthe judgment circuit 1007 are not necessarily configured by anindependent circuit. For example, they may be implemented by anintegrated semiconductor package (microcontroller or custom IC, forexample). The at least a part of the measurement circuit 1006 and thejudgment circuit 1007 may be integrated into the control circuit 1090.

FIG. 8 also describes multiple functional blocks that the controlcircuit 1090 has. Those functional blocks are a power transmissioncontrol unit 1091, the result storage unit 1093, an oscillation controlunit 1094, and a timing control unit 1095. The control circuit 1090 maybe implemented by a combination of a processor such as a CPU (CentralProcessing Unit) and a computer program stored in a memory. A processorexecuting a command group described in a computer program, a function ofeach functional block illustrated in FIG. 8 is implemented.Alternatively, similar functions may be implemented by hardware such asa DSP (Digital Signal Processor) that incorporates a computer program ina semiconductor circuit. At least a part of the control circuit 1090 andat least a part of the foreign substance detection circuit 1004 may beimplemented by a semiconductor package.

The power transmission control unit 1091 performs control related topower transmission. The power transmission circuit 1000 operates whilealternately switching power transmission mode using the inverter circuit1001 and foreign substance detection mode using the foreign substancedetection circuit 1004. In power transmission mode, the powertransmission control unit 1091 inputs a gate pulse of a predeterminedfrequency into each switching element in the inverter circuit 1001. Thiscontrols an AC voltage outputted from the inverter circuit 1001. A powertransmission frequency in this embodiment may be set to a value rangingfrom 100 kHz to 200 kHz, for example. A power transmission frequency maybe set to any value out of the range.

The oscillation control unit 1094 drives the oscillation circuit 1003 inforeign substance detection mode. As described in detail below, theoscillation control unit 1094 oscillates the oscillation circuit 1003 ata first frequency f1 which is lower than the resonance frequency fr ofthe power reception resonator 1010 b and at a second frequency f2 whichis higher than the resonance frequency fr. When the oscillation circuit1003 has the configuration illustrated in FIG. 9, switching of thefrequencies f1, f2 is performed by switching a conduction state of aswitch S3. The frequency fr may be set to approximately 1000 kHz, forexample. The frequency f1 may be set to a value in the range of 400 kHzto 800 kHz, for example. The frequency f2 may be set to a value in therange of 1200 kHz to 1500 kHz. The frequencies fr, f1, f2 are notlimited to this example, and acceptable as far as they satisfy f1<fr<f2.In the embodiment, while fr is higher than a power transmissionfrequency, it may be set to the power transmission frequency or lower.

The timing control unit 1095 controls timing to transmit electric powerand timing to perform a process for foreign substance detection. Thetiming control unit 1095 controls each unit in the power transmissioncontrol circuitry 1090 so that foreign substance detection is performedregularly during charging. After a power transmission process continuesfor a predetermined period of time (several seconds to several tens ofseconds, for example), the timing control unit 1095 stops powertransmission and starts the process for foreign substance detection.When some processes included in the foreign substance detection processcomplete, the timing control unit 1095 resumes the power transmissionprocess. A period of time from when one power transmission process stopstill when a next power transmission process starts (more specifically, aforeign substance sensing session) is controlled to several msec toabout several tens msec, for example. Such control enables foreignsubstance detection without interrupting power transmission for a longtime.

Length of one foreign substance sensing session (more specifically, adivided session in which a process divided in a series of multipleprocesses is performed) is set to a value shorter than a delay periodset for a power reception device, for example. As described above,length of a delay period is a period of time from when powertransmission is stopped till when a notification unit (indicator such asa lamp, for example) of the power reception device notifies the stop ofthe power transmission. This length may be a fixed value or may varydepending on a model of a power reception device. For example, thereception circuit 1005 may receive from the power reception deviceinformation indicating a delay period from when power transmission isstopped till when the stop of power transmission is notified by means ofthe notification unit of the power reception device. In such aconfiguration, the divided session in which each process divided from aseries of multiple processes is performed may be set shorter than thedelay period indicated by the received information.

The timing control unit 1095 further controls the foreign substancedetection circuit 1004 so that foreign substance detection starts attiming when the reception circuit 1005 completes reception of a packetof a feedback signal. With this, loss of a packet can be prevented bystarting a foreign substance detection operation during reception of apacket.

FIG. 10 is a diagram schematically illustrating timing of charging,timing of foreign substance sensing, and timing of packet reception.FIG. 10(a) illustrates an example of a case in which timing control isnot performed. FIG. 10(b) illustrates an example of a case in whichtiming control is performed. During charging operation, the powertransmission circuit 1000 receives a packet of a feedback signal whichis irregularly sent from the power reception circuit 1020. Asillustrated in FIG. 10(a), when the timing of packet reception overlapsthat of foreign substance sensing, a packet may not be receivednormally. This is because in transmission and reception of a packet witha load modulation method, a packet is received by reading a signalcomponent included in waveform of electric power to be transmitted. Whenthe transmitted electric power rapidly changes as power transmission isstopped, a change in the signal component becomes smaller than a changeof the electric power itself, which makes detection difficult.

In this embodiment, in order to avoid the problem described above, thetiming control unit 1095 controls timing of foreign substance sensing.Specifically, the timing control unit 1095 controls each unit so that itdoes not perform foreign substance detection while receiving a packet.For example, as illustrated in FIG. 10(b), when the timing of packetreception overlaps that of stop of power transmission, start of foreignsubstance detection can be delayed by extending a charging period. Whenthe timing control unit 1095 performs such control, it may shift toforeign substance detection at timing when reception of packetcompletes.

As described above, the foreign substance detection process in thisembodiment includes a process to measure a change in a physical quantitysuch as a voltage or inductance, a frequency, or the like (measurementprocess), a process to perform calculation (computation) based on themeasured physical quantity (computation process), and a process todetermine on presence or absence of a foreign substance from thecomputed value (determination process).

An example of a foreign substance detection process in this embodimentis described hereinafter.

<Foreign Substance Detection 1: Coupling Coefficient>

In this embodiment, a coupling coefficient of the detection resonator1011 and the power reception resonator 1010 b can be determined todetect a foreign substance based on a value thereof.

FIG. 11 is a diagram illustrating an operating principle of a couplingcoefficient estimation method used in foreign substance detection inthis embodiment. Suppose that a detection coil L1 (an inductance valueis also noted as L1) and a power reception coil L2 (an inductance valueis also noted as L2) resonating at a frequency fr are coupledelectromagnetically by a coupling coefficient k. Then, input inductanceLin viewed from the detection coil is determined with the followingexpression:Lin(f)=L1{1−k ²/(1−(fr/f)²)}  Expression 1

FIG. 11 is a graph schematically illustrating the expression 1.

It seems that at the frequency f<<fr, both ends of the power receptionresonator 1010 b are substantially open. An input inductance valuemeasured at a first frequency f1 which is lower than fr is Lin(f1). Onthe other hand, it seems that at the frequency f>>fr, both ends of aparallel condenser at the power reception resonator 1010 a aresubstantially shorted. An input inductance value measured at a secondfrequency f2 which is higher than fr is Lin(f2).

When magnitude of f1 and f2 are appropriately set, the followingapproximate expression is obtained from the expression 1:Lin(f1)≈L1Lin(f2)≈L1(1−k ²)

The following expression 2 is obtained from these two approximateexpressions:k ²≈1−Lin(f2)/Lin(f1)  Expression 2

With this expression 2, the coupling coefficient k can be computed basedon a ratio of Lin(f1) and Lin(f2) which are measured values. However,the expression 2 is based on a special condition that the followingexpressions 3 and 4 are true between input inductance Lin_open(f) whenthe power reception coil ends are completely opened and input inductanceLin_short(f) when the power reception coil ends are shorted:Lin_open(f1)−Lin_open(f2)  Expression 3Lin_short(f1)=Lin_short(f2)  Expression 4

To put it another way, if a wireless power transmission system isdesigned after selecting appropriate frequencies f1 and f2 that make theexpressions 3, 4 true, the expression 2 is true, which enablesestimation of the coupling coefficient k. In usual, there is nopractical issue if these frequencies f1, f2 are set in a frequency rangein which dimensions of a resonator are sufficiently smaller thanwavelength.

Note that use of a self-exciting oscillation circuit enables directconversion of a change in input inductance into a change in anoscillatory frequency. More specifically, since an inverse of a squareof an oscillatory frequency dictates input inductance, the couplingcoefficient can be rewritten by the following expression:k ²≈1−f1² /f2²  Expression 5

In practice, since a linear/non-linear element of a circuit is included,the expressions 2 and 5 need to be corrected. However, in principle, thecoupling coefficient k can be estimated from these expressions (Detailsof a correction example is described below with reference to FIG. 12).

With the above description, if an input inductance value in twofrequencies or an oscillatory frequency are measured while continuouslyswitching operations to oscillate at each of the frequencies f1 and f2,the coupling coefficient k can be estimated from a measurement result.The coupling coefficient k varies depending on a shield state of amagnetic field due to a foreign substance between transmission andreception coils. Therefore, for example, when an estimated couplingcoefficient k is below a predetermined threshold, it can be determinedthat a foreign substance is present between the power reception coil andthe power transmission coil.

An example for implementing foreign substance detection based on theprinciple described above is described hereinafter.

FIG. 12 is a diagram illustrating a specific circuit configurationexample of a power transmission device 100 and a power reception device200 in this embodiment. In this example, the load 1040 is a secondarybattery, and the measurement circuit 1006 and the judgment circuit 1007are implemented by a microcontroller (microcontroller). A displayelement 1070 for notifying a user of a foreign substance detectionresult is mounted in the power transmission device 100. The displayelement 1070 may be an LED light source or a display. In the exampleillustrated in FIG. 12, the power transmission resonator 1010 a alsofunctions as the detection resonator 1011. Thus, a power transmissionresonator and a detection resonator may be a common resonator. The powertransmission device 100 includes a switch 1002 for switching aconnection state of the oscillation circuit 1003 and the powertransmission resonator 1010 a.

The power transmission resonator 1010 a has a power transmission coil L1and a condenser C1 connected in series with the power transmission coilL1. The power reception resonator 1010 b has a power reception coil L2,a condenser C2 p connected in parallel with the power reception coil L2,and a condenser C2 s connected in series with the power reception coilL2.

In this example, outside diameter of the power transmission coil L1 isset to 39 mm and inductance is set to L1=13.6 μH. Outside diameter ofthe power reception coil L2 is set to 34 mm and inductance is set toL1=15.8 μ. Capacity of a series capacitor C1 is set to 180 nF, andcapacity of a series capacitor C2 s and that of a parallel capacitor C2p are respectively set to C2 s=120 nF and C2 p=1590 pF. The powertransmission coil L1 resonates at 100 kHz and the power reception coilresonates at 115 kHz and 1000 kHz.

The power transmission coil L1 is connected to the oscillation circuit1003 by way of a selector switch 1002 including switches S1, S2. Theselector switch 1002 electrically shuts off the power transmission coilL1 and the oscillation circuit 1003 in a power transmission session. Theselector switch 1002 electrically connects the power transmission coilL1 and the oscillation circuit 1003 in a foreign substance sensingsession. In the foreign substance sensing session, the inverter circuit1001 is stopped.

The oscillation circuit 1003 in this embodiment is a pierce oscillationcircuit that functions as a self-exiting LC oscillation circuit. Aresistance Rf and a resistance Rd that the oscillation circuit 1003 hasare elements to adjust excitation level of a circuit. The oscillationcircuit 1003 further includes an adjustment inductor Lm and a switch S3for changing an oscillatory frequency. Values of Lm and C11, C12 aredetermined so that oscillation takes place at two frequencies f1—400 kHz(S1 and S2 on, S3 off) and f2—1500 kHz (S1 and S2 on, S3 on) which aredifferent from resonance frequencies of the power reception coil fr=115kHz, fr=1000 kHz. C1 and C2 s seem shorted at f1 and f2 and C2 p seemopen at f1 and shorted at f2. Thus, it may be considered that a maincondenser involved in estimation of a coupling coefficient is C2 p. Inaddition, an estimation expression of a coupling coefficient in thiscircuit configuration example is the following expression (expression 6)in which correction is made on the expression 5:k ²≈1−f1² /f2²/(f2² −f3²)  Expression 6

An oscillatory frequency f3 is an oscillatory frequency when S1 and S2are turned off and S3 is turned on. More specifically, measuring thefrequency f3 is equivalent to measuring an inductance value of theadjustment inductor Lm. When the power transmission coil L1 oscillatesat the frequency f2, the oscillatory frequency includes a componentbased on an input inductance value of the power transmission coil L1 anda component based on an inductance value of the adjustment inductor Lm.Thus, the expression 6 removes effect of the adjustment inductor Lm inthe denominator in the second term and then computes a couplingcoefficient. In this way, the measurement circuit 1006 may detect aforeign substance based on the coupling coefficient k to be computed bythe correction formula 6 based on the expression 5, instead of theexpression 5. Note that a correction formula is not limited to theexpression 6 since various circuit topology exist in the self-excitingLC oscillation circuit, as described earlier. For example, anoscillatory frequency can be changed by switching the condensers C11,C12 in FIG. 12 to a different condenser. In that case, a correctionformula differs from the expression 6. Even when a circuit topology thatdiffers from the above is adopted, derivation of a correction formula ofthe expression 5 is easy. Similarly, when the expression 2 is used, thecoupling coefficient k may be computed by using a correction formulathat is corrected according to a circuit topology. In addition, a valueof each parameter described above is an example, and may be set to avalue different from the above. An important point is that impedancez2−1/jωC2 p of the power reception resonator 1010 is relatively large atthe frequency f1 and relatively small at the frequency f2. Here, j is animaginary number unit, w is an angular frequency, and a relation ofω=2πx frequency is established.

Then, a flow of a foreign substance detection process based on acoupling coefficient is described with reference to a flowchart in FIG.13.

First, when the control circuit 1090 senses approaching of the powerreception resonator 1010 b to the detection resonator 1011, it startsforeign substance sensing mode. Sensing of “approach” in this embodimentis not based on the operating principle of foreign substance sensingdescribed above. Sensing of “approach” may be performed by detecting achange in an oscillatory frequency or a voltage, for example. When thepower reception resonator 1010 b approaches the detection resonator1011, an oscillatory frequency may increase or amplitude of a voltageoutputted from the oscillation circuit 1003 may drop, due to effect ofmetal (board grand or coil, or the like) in the power receptionresonator 1010 b. In addition, when the power reception coil L2 in thepower reception resonator 1010 b includes an electromagnetic shield(magnetic material) for reducing effect of electromagnetic noise on aperipheral circuit, an oscillatory frequency may drop as the powerreception resonator 1010 b approaches. Therefore, detection of a changein an oscillatory frequency or voltage enables approaching of the powerreception resonator 1010 b to be sensed. The oscillation control unit1094 and the oscillation circuit 1003 may be such configured that theyperform intermittent oscillation (intermittent operation) whichoscillates ACs worth of several sessions only once in 1 msec to a fewseconds and that they switch to continuous operation only when theysense approach of the power reception coil L2. Approach of the powerreception coil L2 can be sensed by performing such an intermittentoperation, while controlling an increase in power consumption. Anoperating frequency of the oscillation circuit 1003 in this intermittentoperation may be an arbitrary frequency.

Then, in step S600, the oscillation control unit 1094 causes theoscillation circuit 1003 to operate at the frequency f1.

In step S601, the measurement circuit 1006 measures input inductance anda voltage after a predetermined period of time elapses.

In step S602, the oscillation control unit 1094 causes the oscillationcircuit 1003 to operate at the frequency f2.

In step S603, the measurement circuit 1006 measures input inductance anda voltage after a predetermined period of time elapses.

In step S604, the judgment circuit 1007 computes a coupling coefficientfrom a series of measurement results with the expression 2. In stepS605, the judgment circuit 1007 judges whether or not a voltage hasexceeded a predetermined first threshold. The first threshold may be setto a numeric value in the range of 0.3 to 0.5, for example. When thecomputed coupling coefficient k exceeds the first threshold, it isdetermined that no foreign substance is present between the powerreception coil L2 and the power transmission coil L1. In this case, thejudgment circuit 1007 stores in the result storage unit 1093 informationindicating accordingly. The oscillation control unit 1094 stopsoscillation of the oscillation circuit 1003 based on this information(step S606). Then, a display element such as an LED light source, notshown, mounted in a power transmission device or a power receptiondevice may be caused to emit light or a display of a power receptiondevice may be caused to display that power transmission starts. Withthis, a user can be notified that no foreign substance is presentbetween coils and charging can be performed safely.

Then, the power transmission control unit 1091 drives the invertercircuit 1001 and starts wireless power transmission. Note that wirelesspower transmission may be started not immediately after oscillation ofthe oscillation circuit 1003 is stopped, but, for example, after it isconfirmed that variations in a frequency stops, by a user placing apower reception device on a power transmission device, or the like.

On the other hand, in step S605, when the coupling coefficient k doesnot exceed the predetermined first threshold, the display element may becaused to flash or the display element may display that a foreignsubstance is present. With this, a user can be informed that a foreignsubstance is present between coils and power transmission is dangerous.

In addition, although the coupling coefficient k is computed by theexpression 2 here, the coupling coefficient may be computed by theexpression 5. Instead, the coupling coefficient k may be computed by thecorrection formula of the expression 2 or the expression 5 as describedabove.

Through the foregoing operation, a foreign substance located near thepower transmission coil L1 and the power reception coil L2 can bedetected, and information indicating the detection result can beoutputted. This enables a user to know whether power transmission can beperformed safely.

Note that the operation in this embodiment is not limited to theoperation illustrated in FIG. 13. For example, the judgment process instep S605 is evaluated not only by an absolute quantity of whether ornot a predetermined coupling coefficient k is exceeded but also bywhether or not a temporal change amount of the coupling coefficient k issufficiently small. In addition, detection of a foreign substance may beperformed based on other physical quantity, in addition to the couplingcoefficient k.

<Foreign Substance Detection 2: Input Inductance and Voltage>

Detection of a foreign substance can also be performed based on an inputinductance of a detection coil or a voltage outputted from anoscillation circuit 1003. An example of such a foreign substancedetection process is described hereinafter.

FIG. 14 is a flowchart illustrating a foreign substance detectionprocess in this example. In this example, the foreign substance sensingjudgment circuit 1008 detects a foreign substance through differentthree-phase processes. Here, input inductance of the detection resonator1011, an output voltage of the oscillation circuit 1003 at the frequencyf1, and an output voltage of the oscillation circuit 1003 at thefrequency f2 are selected as a physical quantity (parameter) to bemeasured. The foreign substance sensing judgment circuit 1008 judgeswhether or not each is below a predetermined threshold. With this, aforeign substance can be detected with high accuracy, without dependingon properties or position of a foreign substance.

Three-stage steps (Steps 1 to 3) in this embodiment are described indetail hereinafter.

<Step 1>

When metal that shields the magnetic field is present between a powertransmission coil and a power reception coil, an electric current of aninverse phase to the coils flows on the metal surface, thus reducinginput inductance of the coils. Thus, when the input inductance of thecoils fall below a threshold, it can be judged that a foreign substanceis present. However, since a coupling coefficient changes depending on acombination of a detection coil and a power reception coil, an amount ofinductance to be reduced varies. Therefore, a foreign substance betweencoils can also be detected in a coil pair of a different combination bymaking an inductance threshold Lth a function of a coupling coefficientk. In the example illustrated in FIG. 14, when the input inductance ofthe coil is below a predetermined threshold, it is judged that a foreignsubstance which easily shields the magnetic field (ring-shaped metalforeign substance, for example) is present. In contrast, when the inputinductance of the coil exceeds the predetermined threshold, it is judgedthat such a foreign substance is not present.

<Steps 2 and 3>

When metal that does not easily shield the magnetic field (iron or thelike, for example) is present between the power transmission coil andthe power reception coil, the magnetic field passes through the foreignsubstance and thus a coupling coefficient does not fall easily. Hence,sensing is difficult with the method described above. In such a foreignsubstance, however, an eddy current is generated when the magnetic fieldpasses through the foreign substance and a voltage drops at a coil end.Thus, amplitude of oscillation waveform (voltage) drops. Therefore, whenamplitude of a coil-end voltage is below a predetermined threshold, itcan be judged that a foreign substance is present. However, since acoupling coefficient differs depending on a combination of a detectioncoil and a power reception coil, an amount of voltage to be reducedvaries. Thus, a voltage threshold Vth is made a function of the couplingcoefficient or a function of inductance Lin (or an oscillatory frequencyf). With this, a foreign substance between coils can be detected in acoil pair of a different combination.

In Step 2, the judgment circuit 1007 judges whether or not a coil-endvoltage when the oscillation circuit 1003 oscillates at the frequencyf1, which is smaller than the resonance frequency fr, is equal to orless than a predetermined threshold. When a pair of resonators iscoupled at a frequency lower than the resonance frequency fr, magneticflux in the vicinity of the center of the power transmission coil andthe power reception coil is dense (odd mode). Therefore, in this case,sensitivity of detecting a foreign substance that is present in thevicinity of the center of the power transmission coil and the powerreception coil is high. Hence, in Step 2, it can be detected whether ornot a metal foreign substance such as iron that does not easily shieldthe magnetic field is present in the vicinity of the center of the powertransmission coil and the power reception coil.

In Step 3, the judgment circuit 1007 judges whether or not a voltage atthe coil end when the oscillation circuit 1003 oscillates at thefrequency f2, which is larger than the resonance frequency fr, is equalto or less than the predetermined threshold. When the pair of resonatorsis coupled at a frequency higher than the resonance frequency fr,magnetic flux in a peripheral area away from the center of the powertransmission coil and the power reception coil is dense (even mode).Therefore, in this case, the sensitivity of detecting a foreignsubstance that is present in the peripheral area away from the center ofthe power transmission coil and the power reception coil is high. Hence,in Step 3, it can be detected whether or not a metal foreign substancesuch as iron that does not easily shield the magnetic field is presentin the peripheral area away from the center of the power transmissioncoil and the power reception coil.

In this example, while the processes are performed in the order of Step1 to Step 3, the order of these steps may be changed. In addition, onlya part of these steps may be performed.

<Method for Setting a Threshold>

An idea of threshold determination is described hereinafter.

When a judgment is made on presence or absence of a foreign substancebased on multiple parameters, as described above, there are variousmethods for setting a threshold, as illustratively illustrated in FIG.15 to FIG. 16. As described above, parameters include inductance,resistance, a Q value of a coil, or a frequency or a voltage value thatcan be obtained from conversion of these parameters, or the like. Anexample of setting a threshold when two parameters P1, P2 are selectedtherefrom is described in the following.

FIG. 15 is a diagram illustrating an example in which a threshold of theparameter P2 is a linear function of the parameter P1. This case has theeffect that a judgment process is simple, thus being able to alleviatecalculation load. In addition, as illustrated in FIG. 6, until P1reaches a certain value, the threshold of P2 may be set to a fixedvalue, and when P1 exceeds that value, the threshold of P2 may be set tothe linear function of P1. Alternatively, the threshold of P2 may be alinear function that varies depending on a range of P1. A threshold thusbeing linearly set in multiple stages, the accuracy of detecting aforeign substance can be improved while alleviating the calculationload. As illustrated in FIG. 17, a threshold may be set independentlyfor each of the parameters P1 and P2. This can further make judgmentsimple. Alternatively, as illustrated in FIG. 18, a method can also beconceived that a combination of values of the parameters P1 and P2 in acase in which a foreign substance is present and a case in which noforeign substance is present are retained in advance as a table value ina memory of the control circuit. This method has an advantage thatjudgment can be reliably made on presence or absence of a foreignsubstance although memory usage increases.

Next, an example of division of a foreign substance detection process inthis embodiment is described. As described earlier, the foreignsubstance sensing judgment circuit 1008 in this embodiment detects aforeign substance in the vicinity of the detection resonator 1011 byperforming the foreign substance sensing process including multiplesteps. These multiple steps may include, for example, a step ofmeasuring a physical quantity such as the above-mentioned inputinductance or voltage, frequency, or the like, a step of computing otherphysical quantity such as a coupling coefficient or the like through acalculation based on the measured physical quantity, and a step ofjudging presence or absence of a foreign substance through comparison ofthese physical quantities with a predetermined threshold. Measurement orcalculation of multiple different physical quantities may be dividedinto multiple steps and performed. The foreign substance sensingjudgment circuit 1008 performs different steps in the foreign substancedetection process before or after one power transmission process.

<Process Division Example 1: Division According to a Foreign SubstanceType>

FIG. 19 is a diagram illustrating a first example of division of processin the embodiment 1. In this example, a process is divided according toa type of a foreign substance to be detected. Here, a case is assumed inwhich a foreign substance made of aluminum (aluminum foreign substance)and a foreign substance made of iron (iron foreign substance) aredetected at different timing. Note that a foreign substance type is notlimited to this example.

FIG. 19(a) illustrates an example of a case in which an aluminum foreignsubstance and an iron foreign substance are sensed altogether. FIG.19(b) illustrates a case in which sensing of an aluminum foreignsubstance and sensing of an iron foreign substance are divided. Asillustrated in FIG. 19(a), when an aluminum foreign substance and aniron foreign substance are detected in batch, processing takes muchtime. In contrast to this, as illustrated in FIG. 19(b), when adetection process of an aluminum foreign substance and a detectionprocess of an iron foreign substance are divided, individual processingtime is shorter than the case of batch processing.

FIG. 20A is a diagram showing results of measurements of a couplingcoefficient and an input inductance value in each of a case in which analuminum foreign substance is present and a case in which an aluminumforeign substance is not present, using seven models of evaluationterminals including a power reception coil (φ22 mm to 40 mm) and a powertransmission coil (φ43 mm). The power reception coil of the seven modelsof evaluation terminals is connected to a parallel condenser and aresonance frequency fr which is determined depending on a powerreception coil and the parallel condenser is set to 1000 kHz. Anoscillation circuit is a self-exciting pierce oscillation circuit. Here,a ring (φ22 mm) made of aluminum that shields the magnetic field isselected as an aluminum foreign substance for evaluation.

As shown in FIG. 20A, when an aluminum foreign substance is present,inductance is smaller than a threshold T0 which is a linear function ofa coupling coefficient. Thus, an aluminum foreign substance can bedistinguished based on a coupling coefficient and inductance.

FIG. 20B is a diagram showing results of measurements of a couplingcoefficient and a coil-end voltage in each of a case in which an ironforeign substance is present and a case in which an iron foreignsubstance is not present, under the same conditions as above. Here, aniron disk (φ15 mm) that does not easily shield the magnetic field isselected as an iron foreign substance for evaluation. For the case inwhich a foreign substance is present, a position of the iron disk offsetfrom the center of the power transmission coil is set to four types of 0mm, 5 mm, 10 mm, 15 mm.

As shown in FIG. 20B, when an iron foreign substance is present, thecoil-end voltage is smaller either of thresholds T1, T2. Therefore, aniron foreign substance can be distinguished based on a couplingcoefficient and a voltage.

In this way, a physical quantity necessary for judgment varies dependingon a type of a foreign substance to be detected. For this reason, inorder to detect multiple foreign substances, a necessary physicalquantity is larger and processing takes much time. In this embodiment, aphysical quantity to be measured in one process is reduced by dividing adetection process for each target foreign substance, thus shorteningprocessing time.

Note that for a coupling coefficient necessary for detection of bothaluminum and iron, a value determined in detection of one may be used indetection of the other. For example, a value of a coupling efficientacquired for an aluminum detection process may be stored in the resultstorage unit 1093 and judgment on presence or absence of a foreignsubstance may be made by measuring only a voltage when detecting an ironforeign substance. This can shorten the processing time when detectingan iron foreign substance.

Thus, according to this embodiment, not only a foreign substance can bedetected with high accuracy even during charging through the use of anoscillation circuit that operates at a frequency different from a powertransmission frequency, but also processing time taken for one detectionprocess can be shortened. Therefore, interruption time of charging canbe controlled.

<Process Division Example 2: Division According to a Frequency>

A second example of process division is described hereinafter. In thisexample, processes are divided according to multiple frequencies used inforeign substance detection. Multiple frequencies are, for example, thefrequencies f1 and f2 in the above description.

FIG. 21(a) illustrates an example in which a process at the frequency f1and a process at the frequency f2 are performed consecutively. FIG.21(b) illustrates a case in which the process at the frequency f1 andthe process at the frequency f2 are divided. As described above, acoupling coefficient can be estimated by causing the oscillation circuit1003 to operate at two frequencies f1 and f2 and based on a ratio ofinput inductance of a detection coil in a state in which it oscillatesat each of the frequencies. In order to perform the processes using thetwo frequencies in batch, however, an oscillatory frequency of theoscillation circuit 1003 needs to be switched halfway. Thus, processingcannot be performed in parallel. Since these processes are seriallyprocessed, it takes much time. Thus, as illustrated in FIG. 21(b),separation of processes for every frequency can make the interruptiontime during charging shorter than batch processing.

In this example, the measurement circuit 1006 stores in the resultstorage unit 1093 a measurement result of when the oscillation circuit1003 oscillates at the frequency f1. When a measurement result of whenthe oscillation circuit 1003 oscillates at the frequency f2 is obtained,the judgment circuit 1007 reads the result stored in the result storageunit 1093. The judgment circuit 1007 computes a coupling coefficient byusing both results and calculating. This coupling coefficient can beused in detection of the aluminum foreign substance or the iron foreignsubstance as described above.

Note that in this example, calculation is not performed when themeasurement process at the frequency f1 ends, which is non-limitingexample. When the measurement process at the frequency f1 is complete,the judgment circuit 1007 may read the last measurement result at thefrequency f2 from the result storage unit 1093 to calculate a couplingcoefficient. This can shorten time to measure a physical quantity at thefrequency f2.

<Process Division Example 3: Measuring a Same Physical Quantity MoreThan Once>

A third example of division of processes is described hereinafter. Inthis example, in order to improve the detection accuracy, a process offoreign substance detection in which a same process is performed morethan once is divided.

FIG. 22 is a diagram illustrating this division example. FIG. 22(a)illustrates an example in which a measurement process A for one physicalquantity is performed twice in a row. FIG. 22(b) illustrates a case inwhich the measurement process A is divided to each session. Themeasurement process A represents a process to measure one physicalquantity of a voltage, an input inductance, a frequency or the like inthe description above. In this example, the measurement process A isperformed twice, which is a non-limiting example. Instead, themeasurement process A may be performed three times or more untilaccuracy can be ensured. Performing the measurement process A more thanonce and taking a mean of measurement results improve the accuracy ofthe measurement result. However, if the number of times of repetition isincreased to improve the accuracy, the stop time becomes longeraccordingly. Thus, if a measurement process is divided, as illustratedin FIG. 22(b), the processing time per session can be shortened whileensuring the accuracy of detection. Consequently, the interruption timeof power transmission per session can be shortened.

A process division method is not limited to this and various aspects arepossible. If a measurement process and a calculation process are dividedinto multiple steps and results in the middle of the processes arestored in the result storage unit 1093, the results can be called tomake a judgment when necessary physical quantities are ready.

<Process Division Example 4: Changing Frequency>

A fourth example of process division is described hereinafter. In thisexample, when detecting a foreign substance by performing a measurementprocess A and a measurement process B, the foreign substance detectioncircuit 1004 changes frequency of the processes according to time takenfor these processes. The process A and the process B may be a process tomeasure two different physical quantities in the description above.Here, it is assumed that time taken for the measurement process A isshorter than time taken for the measurement process B.

FIG. 23(a) illustrates an example in which the process A and the processB are performed alternately. FIG. 23(b) illustrates a case in which thefrequency of the process B whose processing time is longer is made lowerand the frequency of the process A whose processing time is shorter ismade higher. As illustrated in FIG. 23(b), the increased frequency ofthe process A whose processing time is shorter enables further reductionof the charging interruption time.

In this example, the measurement circuit 1006 stores measurement resultsin the memory 1093 after the measurement processes A, B. The judgmentcircuit 1007 calls the result of last measurement process B from thememory 1093 after the measurement process A, and performs a calculationprocess to judge on presence or absence of a foreign substance. Afterthe measurement process B, the judgment circuit 1007 calls the result ofthe last measurement process A after the measurement process B, andperforms a calculation process to judge presence or absence of a foreignsubstance.

With the above processing, the foreign substance sensing time withrespect to entire charging time can be shortened. In addition, when aresult different from that of the last measurement process A is obtainedin a measurement process A, it is likely that a foreign substance hasmixed into coils. Thus, even if a scheduled next process is A, theprocess B may be performed.

<Process Division Example 5: Omitting Calculation>

A fifth example of process division is described hereinafter. In thisexample, a foreign detection process is divided to a measurement processand a calculation process for every physical quantity to be measured. Ifa measurement result of a physical quantity is same as a result of lasttime, the foreign substance detection circuit 1004 omits the calculationprocess and makes same judgment as the last time.

FIG. 24 is a diagram illustrating a relation of processes related to acharging process and a process related to foreign substance detection inthis example. In this example, a measurement process is divided toprocesses A, B, C, and a calculation process is divided to processes A′,B′, C′. After performing the processes A, A′, B, B′, C, C′ in thisorder, the foreign substance detection circuit 1004 judges presence orabsence of a foreign substance.

If it is finally judged that there is no foreign substance after thecalculation process C′, charging continues to be performed. When aresult of the measurement process A in a next session does notsubstantially differ from the result of the last measurement process A,the foreign substance detection circuit 1004 does not perform thecalculation process A′ and immediately judges that there is no foreignsubstance. With this, the calculation process after the measurementprocess A can be omitted. Similarly, when a result of a measurementprocess B or C does not substantially differ from the result of the lastmeasurement, the foreign substance detection circuit 1004 also judgesthat there is no foreign substance. The processing described above canshorten the interruption time during charging.

In this example, when each measurement process is complete, themeasurement circuit 1006 stores a value thereof in the memory 1093.During a next measurement, the judgment circuit 1007 reads the measuredvalue stored in the memory 1093 and compares both values. When thejudgment circuit 1007 determines that a difference between both valuesdoes not substantially differ, it judges that there is no foreignsubstance.

As illustrated in FIG. 25, when there is no change in a measurementvalue in the process A, the process B and the process C may be omitted.In this case, only the process A is performed till any change occurs inmeasurement values in the processes A, the calculation process A′ andother processes B, B′, C, C′ are performed when a change has occurred,and the judgment circuit 1007 may judge on presence or absence of aforeign substance.

<Process Division Example 6: Division of Sensing and Calculation>

A sixth example of process division is described hereinafter. In thisexample, a measurement process and a calculation process are separatedand the calculation process is performed during a period of charging.The measurement process is a process to measure a physical quantity of avoltage, a frequency, input inductance or the like. The calculationprocess is a process to judge on presence or absence of a foreignsubstance by calculation based on a measured physical quantity.

FIG. 26(a) illustrates a case in which a measurement process and acalculation process are consecutively performed while charging isinterrupted. FIG. 26(b) illustrates a case in which a calculationprocess is performed in a next period of charging. In this example, asillustrated in FIG. 26(b), since the calculation process is performedduring charging, power transmission has only to be stopped only for timein which the measurement process is performed. Thus, one charginginterruption time can be shortened. Consequently, a proportion of timein which charging is performed to the entire charging period increases.

As described above, the examples of process division in this embodimenthave been described. However, a method for dividing a process is notlimited thereto. For example, when a coupling coefficient k is not used,there is no need to divide a process to multiple frequencies and processthem. This disclosure is not limited to any specific method, as far as aseries of multiple processes included in a foreign substance detectionprocess is divided among multiple foreign substance detection sessionswhen the foreign substance detection process is performed based on aphysical quantity that varies depending on approach of a foreignsubstance to the power transmission resonator.

(Embodiment 2)

A second embodiment is described hereinafter. This embodiment differsfrom the embodiment 1 in that a detection resonator (third resonator)1011 to detect a foreign substance and a power transmission resonator1010 a (second resonator) are a common resonator. Differences from theembodiment 1 are mainly described in the following.

FIG. 27 is a diagram illustrating a schematic configuration of awireless power transmission system in this embodiment. FIG. 28 is adiagram illustrating a detailed configuration of the control circuit1090 and the foreign substance detection circuit 1004 in FIG. 27. Inthis wireless power transmission system, the power transmissionresonator 1010 a functions as the detection resonator 1011 for detectinga foreign substance 1050. A power transmission circuit 1000 furtherincludes a selector switch 1002. The selector switch 1002 switches acircuit to be connected to the power transmission resonator 1010 a.

FIG. 29 is a diagram illustrating a configuration example of theselector switch 1002. The selector switch 1002 switches a state in whichan inverter circuit 1001 and a power transmission resonator 1010 a areconducting and a state in which an oscillation circuit 1003 and thepower transmission resonator 1010 a re conducting. This switching isperformed by a selector switch control unit 1092 in the control circuit1090.

In a power transmission session, the selector switch control unit 1092electrically connects the inverter circuit 1001 with the powertransmission resonator 1010 a. In a foreign substance sensing session,the selector switch control unit 1092 electrically connects theoscillation circuit 1003 with the power transmission coil 1010 a. Thisenables power transmission and foreign substance sensing to be performedalternately, similar to the embodiment 1.

Also in this embodiment, process division similar to the embodiment 1can be applied. For example, a process in which a measurement circuit1006 measures a physical quantity that varies depending on inputimpedance of the power transmission resonator 1010 a as well as acomputation process and a determination process performed by a judgmentcircuit 1007 can be divided into multiple steps and performed whilepower transmission is interrupted. Since dividing to multiple steps andperforming the foreign substance detection process increases thefrequency of foreign substance detection, thus improving safety.

(Examples)

Examples of this embodiment are described hereinafter. In this example,in the configuration described with reference to FIG. 14, the effect offoreign substance detection is verified using multiple evaluationcircuits. In this example, similar to the embodiment 2, a configurationis adopted in which a power transmission resonator and a detectionresonator are a common resonator.

FIG. 30 to FIG. 32 are diagrams illustrating detections result ofdetermination made on presence or absence of a foreign substance byusing seven models of evaluation terminals that includes a powerreception coil (φ22 mm to 40 mm) which is different from a powertransmission coil (φ40 mm). Here, a determination on presence or absenceof a foreign substance is made based on a flowchart illustrated in FIG.14. The power reception coils of the seven evaluation terminals areconnected with a parallel condenser and a resonance frequency fr whichis dictated by the power transmission coil and the parallel condenser isset to 1000 kHz. The oscillation circuit is a self-exciting pierceoscillation circuit capable of oscillating at a first frequency f1 whichis lower than the resonance frequency fr and at a second frequency f2which is higher than the resonance frequency fr. In this example, as anevaluation foreign substance, a metal ring (φ22 mm) is selected as aforeign substance that shields the magnetic field and an iron disk (φ15mm) as a foreign substance that does not shield the magnetic field areselected.

First, judgment on a presence or absence of a metal ring is madeaccording to Step 1 in FIG. 14. With reference to a measurement resultin FIG. 30(a), it can be seen that inductance tends to decrease as acoupling coefficient becomes lower. It can also be seen that theinductance tends to further decrease if the metal ring is presentbetween the power transmission coil and the power reception coil. Thus,an inductance threshold T0 is set in view of this difference. It isassumed that the threshold T0 is a function which takes the couplingcoefficient as a variable, and that a foreign substance is present whenthe coupling coefficient is equal to or lower than T0. In FIG. 30(b), acase in which the inductance is equal to or lower than T0 is excluded.From a comparison of the cases before and after judgment in FIG. 30, itcan be seen that the case in which the metal ring is present between thepower transmission coil and the power reception coil is reliablyexcluded.

Then, judgment on presence or absence of an iron disk (in the vicinityof the center) is made according to Step 2 in FIG. 14. Evaluation isperformed by setting an offset position of the iron disk from the centerof the power transmission coil to the four types of 0 mm, 5 mm, 10 mm,and 15 mm, and setting a frequency to f1 (odd mode). With reference to ameasurement result in FIG. 31(a), it can be seen that a voltage tends todecrease as a coupling coefficient becomes low. It can also be seen thata coil-end voltage tends to further decrease when an iron disk ispresent between the power transmission coil and the power receptioncoil. Thus, thresholds T1 and T2 of the coil-end voltage are set basedon this difference. It is assumed that the thresholds T1, T2 are afunction which takes a coupling coefficient as a variable and that aforeign substance is present when the coil-end voltage is equal to orlower than T1 or equal to or lower than T2. From a comparison of thecases before and after judgment in FIG. 31, it can be seen that the casein which the iron disk with an offset of 0 mm to 5 mm is present betweenthe power transmission coil and the power reception coil is reliablyexcluded. Since this operation mode is the above-mentioned odd modeoperation, magnetic flux in the vicinity of the coil is dense. Thus, theiron disk with the offset of 0 mm to 5 mm can be mainly detected.

Lastly, according to Step 3 in FIG. 14, judgment on presence or absenceof an iron disk (peripheral area) is made. Evaluation is performed bysetting an offset position of the iron disk from the center of the powertransmission coil to the four types of 0 mm, 5 mm, 10 mm, and 15 mm, andsetting a frequency to f2 (even mode). FIG. 32 shows a result thereof.However, the foreign substance that can be excluded in Step 2 is notshown. With reference to a measurement result of FIG. 32(a), it can beseen that the coil-end voltage tends to decrease as an oscillatoryfrequency increases, that is to say, inductance which is an inverse of apower thereof decreases. It can also be seen that the voltage tends tofurther decrease when the iron disk is present between the powertransmission coil and the power reception coil. Thus, a threshold T3 ofthe coil-end voltage is set based on this difference. It is assumed thatthe threshold T3 is a function which takes the oscillatory frequency asa variable and that a foreign substance is present when the coil-endvoltage is equal to or lower than T3. From a comparison of the casesbefore and that after judgment in FIG. 32, it can be seen that a case inwhich the iron disk with the offset of 10 mm to 15 mm is present betweenthe power transmission coil and the power reception coil can be reliablyexcluded. Since this operation mode is the above-mentioned even modeoperation, magnetic flux on an area from the inner diameter to the outerdiameter of the coil is dense. Thus, the iron disk with the offset of 10to 15 mm can be mainly detected.

By performing the procedure of Step 1 to Step 3 as described above, itcan be confirmed that a foreign substance between coils can be reliablydetected even for a combination of different power transmission coil andpower reception coil. A measurement parameter (voltage, frequency,coupling coefficient) used in foreign substance judgment in this exampleis one example, and similar detection is also possible when otherparameter is used. It is also possible to make judgment on presence orabsence of a foreign substance based on input impedance of a detectionresonator at a frequency f1 which is lower than the above-mentionedresonance frequency and at a frequency f2 which is higher than theresonance frequency, a secondary parameter that is calculated therefrom,or a third parameter that is calculated from a combination thereof. Aselection of these parameters and thresholds may be appropriatelychanged depending on an intended application of a power transmissiondevice and a power reception device including the foreign substancesensing judgment circuit in this disclosure.

In this example, while a description is given on the assumption that thethree steps illustrated in FIG. 14 are performed consecutively, inpractice, the timing control described in the embodiment 1 may beperformed. For example, when the timing control illustrated in FIG. 24is applied, the measurement process and the calculation (determination)process in Step 1 may be made A, A′, the measurement process and thecalculation process in Step 2 B, B′, and the measurement process and thecalculation process in Step 3 C, C′. The timing control is not limitedto this, and the timing control described with reference to FIG. 19,FIG. 21, FIG. 22, FIG. 23, FIG. 25, and FIG. 26 may also be applied.Alternatively, a combination of more than one of the timing control maybe used.

As described, this disclosure includes a power transmission device and awireless power transmission system described in the following items.

[Item 1]

A power transmission device comprising:

-   -   an inverter that generates first AC power and transmits the        first AC power wirelessly to a first resonator of a power        receiving device via a second resonator;    -   an oscillator that generates second AC power which is smaller        than the first AC, and transmits the second AC power to the        first resonator via a third resonator;    -   a foreign substance detector that performs a series of multiple        processes thereby to determine whether or not a foreign        substance is present between the first resonator and the third        resonator based on a physical quantity at the third resonator,        the physical quantity varying depending on the second AC power;        and    -   power transmission control circuitry operative to:    -   cause the foreign substance detector to perform the series of        multiple processes before a start of a transmission of the first        AC power;    -   cause the inverter to start the transmission of the first AC        power if the foreign substance detector determines that the        foreign substance is not present;    -   repeat a foreign substance detection period and a power        transmission period alternately where the foreign substance        detection period is a period in which the foreign substance        detector performs one of the series of multiple processes and        the power transmission period is a period in which the inverter        transmits the first AC power, after the start of the        transmission of the first AC power; and    -   cause the foreign substance detector to divide the series of        multiple processes and determine whether or not the foreign        substance is present as a result of performing all of the        divided series of multiple processes.

According to the aspect described above,

-   -   the power transmission control circuitry    -   causes the foreign substance detector to perform a series of        multiple processes and determine whether or not a foreign        substance is present before transmission of the first AC power        starts, and then causes the inverter circuit to start power        transmission of the first AC power;    -   after the transmission of the first AC power starts, repeats a        foreign substance sensing session in which the foreign substance        sensing is performed and a power transmission session in which        power transmission of the first AC power is performed in such a        way that the series of multiple processes is divided and        performed in the repeated multiple foreign substance sensing        sessions; and    -   causes the foreign substance detector to determine whether or        not a foreign substance is present by dividing and performing        the series of multiple processes using the multiple foreign        substance sensing sessions.

This can make length of the one foreign substance sensing session short(more specifically, power transmission stop time short) and reduce aproportion of power transmission time in which power is transmitted totime of foreign substance sensing. Thus, reduction of the powertransmission efficiency can be prevented. This can also make the oneforeign substance sensing session short (the power transmission stopperiod short). For example, this can make the power transmission stopperiod shorter than a delay period from when power transmission isstopped to when the power transmission stop is notified by means of anotification unit of a power reception device. Thus, a lamp indicatingthat charging is ongoing at the power reception device can be keptlighted.

Then, foreign substance sensing with high accuracy can be performed bycausing the foreign substance detector to determine whether or not aforeign substance is present by performing all the divided processes inthe series of multiple processes.

[Item 2]

The power transmission device of item 1, wherein a division session inwhich a divided process in the series of multiple processes is performedis shorter than a delay period from when power transmission is stoppedto when the power reception device notifies the stop of the powertransmission.

According to the aspect described above,

-   -   the power transmission stop time during power transmission is        stopped can be made shorter than length of the delay period till        the stop of power transmission is notified by means of the        notification unit of the power reception device. Thus, the        notification unit (such as a lamp) indicating that charging is        ongoing can be continuously kept.        [Item 3]

The power transmission device of item 1 or 2, wherein

-   -   the series of multiple processes includes a determination        process to determine that the foreign substance is present        between the first resonator and the third resonator when a        difference between the physical quantity after change and a        predetermined reference value is larger than a preset threshold.

According to the aspect described above,

-   -   the foreign substance sensing session can be made shorter since        there is no process to determine whether or not the foreign        substance is present based on a value computed from the measured        physical quantity.        [Item 4]

The power transmission device of item 1 or 2, wherein

-   -   the series of multiple processes includes:    -   a measurement process to measure the physical quantity at the        third resonator which varies depending on the second AC power;        and    -   a determination process to determine whether or not the foreign        substance is present, based on a value calculated from the        measured physical quantity.

According to the aspect described above,

-   -   division of the measurement process and the determination        process can shorten power transmission stop period, in which        power transmission is stopped, in the foreign substance sensing        period and enables foreign substance sensing with high accuracy        after power transmission starts while avoiding reduction of the        power transmission efficiency.        [Item 5]

The power transmission device according to item 1 or 2, wherein

-   -   the series of multiple processes includes:    -   two or more types of measurement processes to measure physical        quantities at the third resonator which vary depending on the        second AC power; and    -   a determination process to determine whether or not the foreign        substance is present, based on a value computed from the        physical quantities measured at the two or more types of        measurement processes.

According to the aspect described above,

-   -   a foreign substance can be sensed with high accuracy.        [Item 6]

The power transmission device of item 5, wherein

-   -   the two or more types of measurement processes includes a first        type of measurement process in which the foreign substance        detector measures the physical quantity when the oscillation        circuit oscillates at a first frequency f1 which is lower than a        resonance frequency fr of the third resonator, and a second type        of measurement process in which the foreign substance detector        measures the physical quantity when the oscillation circuit        oscillates at a second frequency f2 which is higher than the        resonance frequency fr of the third resonator.

According to the aspect described above,

-   -   a foreign substance can be sensed with high accuracy.        [Item 7]

The power transmission device of item 1 or 2, wherein

-   -   the series of multiple processes includes a first determination        process to measure a first physical quantity corresponding to a        first type of foreign substance and determine whether or not the        first type of foreign substance is present and a second        determination process to determine a second physical quantity        corresponding to a second type of foreign substance and        determine whether or not the second type of foreign substance is        present, and    -   the power transmission control circuitry causes the inverter        circuit to transmit the first AC power between the first        determination process and the second determination process.

According to the aspect described above,

-   -   the first type of foreign substance and the second type of        foreign substance can be sensed with high accuracy.        [Item 8]

The power transmission device of one of items 1 to 7, wherein

-   -   the first resonator and the third resonator are a common        resonator,    -   the power transmission device including a switch that        switches i) electric connection of the inverter circuit and the        common resonator and ii) electric connection of the oscillation        circuit and the common resonator, under control of the power        transmission control circuitry, and    -   when finishing a first process in the divided processes in the        series of multiple processes and resuming power transmission of        the first AC power, the power transmission control circuitry        controls the switch and switches from the electric connection of        the oscillation circuit and the common resonator to the electric        connection of the inverter circuit and the common resonator, and    -   when interrupting the power transmission of the first AC power        and starting a second process following the first process, the        power transmission control circuitry controls the switch and        switches from the electric connection of the inverter circuit        and the common resonator to the electric connection of the        oscillation circuit and the common resonator.

According to the aspect described above,

-   -   the number of components as well as cost can be reduced by        causing the common resonator to act as the first resonator and        the third resonator.        [Item 9]

The power transmission device of one of items 1 to 8, wherein

-   -   a physical quantity at the third resonator is a voltage applied        to the third resonator, a current flowing to the third        resonator, a frequency of the voltage applied to the third        resonator, an input impedance value of the third resonator, or        an input inductance value of the third resonator.

According to the aspect described above,

-   -   it can be easily determined whether or not a foreign substance        is present between the first resonator and the third resonator,        by measuring the physical quantity.        [Item 10]

The power transmission device of one of items 1 to 9, wherein

-   -   the first resonator has a parallel resonance circuit including a        coil and a capacitor,    -   the physical quantity at the third resonator is an input        inductance of the third resonator, and    -   the foreign substance detector:    -   measures an input inductance value Lin(f1) of the third        resonator when the oscillation circuit oscillates at the first        frequency f1 in the first type of measurement process and an        input inductance value Lin(f2) of the third resonator when the        oscillation circuit oscillates at the second frequency f2 in the        second type of measurement process, and    -   calculates a coupling coefficient k in accordance with an        expression of k²=1−Lin(f2)/Lin(f1) to determine based on the        calculated coupling coefficient k whether or not a foreign        substance is present, in the determination process.

According to the aspect described above,

-   -   a coupling coefficient is calculated in accordance with an        expression of k²=1−Lin(f2)/Lin(f1) and it is determined based on        the calculation coupling coefficient k whether or not a foreign        substance is present.

If an input inductance value of the third resonator when both ends ofthe coil are shorted is used for Lin(f2) and an input inductance valueof the third resonator when both ends of the coil are open is used forLin(f1), a coupling coefficient k of high accuracy can be calculated andit can be determined with high accuracy whether or not a foreignsubstance is present.

A parallel resonance circuit including the coil and a capacitor providedat both ends of the coil is provided in the power reception device. Withthis, a state in which both ends of the coil are substantially open canbe created since no electric current flows to the capacitor when theoscillation circuit is driven at the frequency f1 which is lower thanthe second resonance frequency f2. In addition, a state in which bothends of the coil are shorted can be created since the electric currentflows to the capacitor when the oscillation circuit is driven at thefrequency f2 which is higher than the second resonance frequency fr.

Thus, only provision of a capacitor at both ends of the coil makes itpossible to create the state in which both ends of the coil aresubstantially open and the state in which both ends of the coil areshorted. Accordingly, unlike usual practice, there is no need to providea shorting switch at both ends of the coil and provide in the powerreception device a control circuit that controls the provided shortingswitch. Thus, the burden of sending a signal from the power transmissiondevice to control the shorting switch, which has been usually performed,can be removed. Consequently, since foreign substance sensing isperformed with the coupling coefficient of high accuracy, foreignsubstance sensing can be performed with high prevision without resultingin increased cost, even when the load fluctuates in simpleconfiguration.

[Item 11]

The power transmission device of one of items 1 to 10, wherein

-   -   the first resonator has a parallel resonance circuit including a        coil and a capacitor,    -   the physical quantity at the third resonator is an input        inductance of the third resonator; and    -   the foreign substance detector:    -   measures an input inductance value Lin(f1) of the third        resonator when the oscillation circuit oscillates at the first        frequency f1 in the first type of measurement process and an        input inductance value Lin(f2) of the third resonator when the        oscillation circuit oscillates at the second frequency f2 in the        second type of measurement process, and    -   calculates a ratio of the Lin(f1) to the Lin(f2) to determine        based on the calculated ratio whether or not a foreign substance        is present, in the determination process.

What is meant by “based on a ratio of the input inductance value Lin(f1)to the input inductance value Lin(f2)” is described hereinafter.

The expression 1 to calculate the coupling coefficient k of[k²=1−Lin(f2)/Lin(f1)] can be transformed to an expression 2[Lin(f2)/Lin(f1)=1−k²]. Thus, when Lin(f2)/Lin(f1) is determined, acoupling coefficient k can be uniquely determined. Therefore, it can bedetermined based on a ratio of the input inductance value Lin(f1) to theinput inductance value Lin(f2) whether or not a foreign substance ispresent between the first resonator and the third resonator.

According to the aspect described above, in order to calculate acoupling coefficient k with the expression 1, a calculation process of asquare root other than the four arithmetic operations is requested. Onthe other hand, since a ratio of the input inductance value Lin(f1) tothe input inductance value Lin(f2) is a simple division, load ofprocessing can be alleviated and computation speed can be accelerated.

In addition, similar to the aspect described above, there is no need toprovide a shorting switch at both ends of the coil, and thus the burdenof sending a signal from the power transmission device to control theshorting switch can be removed.

[Item 12]

The power transmission device of one of items 1 to 11, wherein

-   -   the first resonator has a parallel resonance circuit including a        coil and a capacitor,    -   the oscillation circuit is a self-exciting circuit,    -   the physical quantity at the third resonator is a frequency of a        voltage applied to the third resonator,    -   a square of an oscillatory frequency of the oscillation circuit        is inversely proportional to an input inductance value of the        third resonator, and    -   the foreign substance detector:    -   measures a frequency f1 of the third resonator when the        oscillation circuit oscillates in the first type of measurement        process and measures a frequency f2 of the third resonator when        the oscillation circuit oscillates in the second type of        measurement process, and    -   calculates a coupling coefficient k in accordance with an        expression of k²=1−f1 ²/f2 ² to determine based on the        calculated coupling coefficient k whether or not the foreign        substance is present, in the determination process.

According to the aspect described above,

-   -   when the oscillation circuit is a self-exciting circuit, and if        it is assumed that the input inductance value is L and the        capacitor is C, the frequency can be represented by the        expression of f=1/(2πx(LC)^(½)). Since capacity C is a circuit        constant and known, and the input inductance value L is        inversely proportional of the square of the frequency of the        oscillation circuit, the expression for the coupling        coefficient, k²=1−Lin(f2)/Lin(f1) can be replaced by the        expression of k²=1−f1 ²/f2 ². With this, a step of measuring the        input inductance with the measurement circuit is eliminated and        vales of frequencies f1 and f2 oscillated by the oscillation        circuit may be used. Thus, since there is no need of measuring        the input inductance with the measurement circuit, the coupling        coefficient can be calculated with high accuracy. Note that for        values of the frequency f1 and the frequency f2, the measurement        circuit may measure the frequency f1 and the frequency f2 of the        first resonator. In addition, a similar idea may also be applied        to other oscillation circuits and can be easily analogized by        those skilled in the art.

Similar to the aspect described above, there is no need to provide ashorting switch at both ends of the coil, and thus the burden of sendinga signal from the power transmission device to control the shortingswitch can be removed.

[Item 13]

A wireless power transmission system, comprising:

-   -   a power reception device that includes    -   a first resonator,    -   a power receiving circuit that converts the first AC received by        the first resonator to first DC power and    -   a load that supplied the first DC power from the power receiving        circuit; and    -   a power transmission device, that includes    -   an oscillator that generates second AC power which is smaller        than the first AC, and transmits the second AC power to the        first resonator via a third resonator,    -   a foreign substance detector that performs a series of multiple        processes thereby to determine whether or not a foreign        substance is present between the first resonator and the third        resonator based on a physical quantity at the third resonator,        the physical quantity varying depending on the second AC power        and    -   power transmission control circuitry operative to:    -   cause the foreign substance detector to perform the series of        multiple processes before a start of a transmission of the first        AC power;    -   cause the inverter to start the transmission of the first AC        power if the foreign substance detector determines that the        foreign substance is not present;    -   repeat a foreign substance detection period and a power        transmission period alternately where the foreign substance        detection period is a period in which the foreign substance        detector performs one of the series of multiple processes and        the power transmission period is a period in which the inverter        transmits the first AC power, after the start of the        transmission of the first AC power; and    -   cause the foreign substance detector to divide the series of        multiple processes and determine whether or not the foreign        substance is present as a result of performing all of the        divided series of multiple processes.        [Item 14]

A foreign substance detecting method using a power transmission device,the power transmission device including:

-   -   an inverter that generates first AC power and transmits the        first AC power wirelessly to a first resonator of a power        receiving device via a second resonator;    -   an oscillator that generates second AC power which is smaller        than the first AC, and transmits the second AC power to the        first resonator via a third resonator;    -   a foreign substance detector that performs a series of multiple        processes thereby to determine whether or not a foreign        substance is present between the first resonator and the third        resonator based on a physical quantity at the third resonator,        the physical quantity varying depending on the second AC power;        and    -   a power transmission control circuitry that controls the power        transmission device,    -   the method comprising causing the power transmission control        circuitry to:    -   cause the foreign substance detector to perform the series of        multiple processes before a start of a transmission of the first        AC power;    -   cause the inverter to start the transmission of the first AC        power if the foreign substance detector determines that the        foreign substance is not present;    -   repeat a foreign substance detection period and a power        transmission period alternately where the foreign substance        detection period is a period in which the foreign substance        detector performs one of the series of multiple processes and        the power transmission period is a period in which the inverter        transmits the first AC power, after the start of the        transmission of the first AC power; and    -   cause the foreign substance detector to divide the series of        multiple processes and determine whether or not the foreign        substance is present as a result of performing all of the        divided series of multiple processes.

According to the aspect described above,

-   -   the power transmission control circuitry is caused to:    -   cause the foreign substance detector to perform a series of        multiple processes to determine whether or not a foreign        substance is present before a transmission of the first AC power        starts and then cause the inverter circuit to start the power        transmission of the first AC power,    -   repeat a foreign substance sensing session in which foreign        substance sensing is performed and a power transmission session        in which the power transmission of the first AC is performed        after the he power transmission of the first AC power starts,        the series of multiple processes being divided and performed in        the repeated multiple foreign substance sensing sessions; and    -   cause the foreign substance detector to divide and perform the        series of multiple processes using the multiple foreign        substance sensing sessions them, and determine whether or not a        foreign substance is present.

This can make length of the one foreign substance sensing session short(more specifically, power transmission stop time short) and reduce aproportion of power transmission time in which power is transmitted totime of foreign substance sensing. Thus, reduction of the powertransmission efficiency can be prevented. This can also make the oneforeign substance sensing session short (the power transmission stopperiod short). For example, this can make the power transmission stopperiod shorter than a delay period from when power transmission isstopped to when the power transmission stop is notified by means of anotification unit of a power reception device. Thus, a lamp indicatingthat charging is ongoing at the power reception device can be keptlighted.

Then, foreign substance sensing with high accuracy can be performed bycausing the foreign substance detector to determine whetheror not aforeign substance is present by performing all the processes divided inthe series of multiple processes.

[Item 15]

A power transmission device comprising:

-   -   an inverter that generates first AC power and transmits the        first AC power wirelessly to a first resonator of a power        receiving device via a second resonator;    -   an oscillator that generates second AC power which is smaller        than the first AC, and transmits the second AC power to the        first resonator via a third resonator;    -   a foreign substance detector that performs a series of multiple        processes thereby to determine whether or not a foreign        substance is present between the first resonator and the third        resonator based on a physical quantity at the third resonator,        the physical quantity varying depending on the second AC power;        and    -   power transmission control circuitry operative to:    -   cause the foreign substance detector to perform the series of        multiple processes before a start of a transmission of the first        AC power;    -   cause the inverter to start the transmission of the first AC        power if the foreign substance detector determines that the        foreign substance is not present;    -   repeat a foreign substance detection period and a power        transmission period alternately where the foreign substance        detection period is a period in which the foreign substance        detector performs one of the series of multiple processes and        the power transmission period is a period in which the inverter        transmits the first AC power, after the start of the        transmission of the first AC power; and    -   cause the foreign substance detector to divide measurements of        the physical quantity including the series of multiple processes        and perform the divided measurement in one of the repeated        foreign substance detection periods;    -   cause the foreign substance detector to divide the rest of the        processes other than the measurements of the physical quantity        including the series of multiple processes and perform the        divided rest of process in parallel with one of the repeated        power transmission periods,    -   determine whether or not the foreign substance is present as a        result of performing all of the divided series of multiple        processes.

According to the aspect described above,

-   -   the power transmission control circuitry:    -   causes the foreign substance detector to perform a series of        multiple processes from measurement of the physical quantity to        determination on the foreign substance, before transmission of        the first AC power starts and determine whether or not a foreign        substance is present, and then causes the inverter circuit to        start power transmission of the first AC power;    -   after the power transmission of the first AC power starts,        repeats a power transmission session in which the foreign        substance sensing is performed and a power transmission session        in which transmission of the first AC power is performed, in        such a way that the measurement of the physical quantity        included in the series of the multiple processes are divided and        performed in the repeated multiple foreign substance sensing        sessions, while the rest of the series of multiple processes        other than the measurement of the physical quantity is divided        and performed in the repeated multiple power transmission        sessions, and    -   causes the foreign substance detector to determine whether or        not a foreign substance is present by dividing and performing        the measurement of the physical quantity using the multiple        foreign substance sensing sessions, and performing the rest of        the processes other than the measurement of the physical        quantity in parallel with the power transmission sessions.

According to the aspect described above, power transmission stop timecan be further shortened and reduction of the power transmissionefficiency can be prevented.

[Item 16]

A wireless power transmission system including:

-   -   a power transmission circuit that is configured to convert        inputted direct current energy into AC energy and output it;    -   a power transmission resonator that is configured to send out        the AC energy outputted from the power transmission circuit;    -   a power reception resonator that is configured to receive at        least some of the AC energy sent out by the power transmission        resonator; and    -   a power reception circuit that is configured to convert the AC        energy received by the power reception resonator to direct        current energy and supply the direct current power to a load,    -   wherein the power transmission circuit has:    -   an inverter circuit that is configured to convert the direct        current energy into the AC energy and output it;    -   a foreign substance detection circuit that is configured to        detect a foreign substance in the vicinity of the power        transmission resonator by performing a foreign substance        detection process including multiple steps; and    -   a control circuit that controls the inverter circuit and the        foreign substance detection circuit so as to alternately repeat        a power transmission process using the inverter circuit and a        process using the foreign substance sensing circuit, and    -   wherein the foreign substance detection circuit is configured to        perform different steps included in the multiple steps before        and after one power transmission process.        [Item 17]

The wireless power transmission system of item 16, wherein the multiplesteps include a step of measuring at least one physical quantity thatchanges as the foreign substance approaches and a step of judging basedon a change amount from a reference value of the at least one physicalquantity.

[Item 18]

The wireless power transmission system of item 17, wherein the multiplesteps include a step of measuring a first physical quantitycorresponding to a first type of foreign substance and a step ofmeasuring a second physical quantity corresponding to a second type offoreign substance.

[Item 19]

The wireless power transmission system of one of items 16 to 18, wherein

-   -   the foreign substance detection circuit has:    -   an oscillation circuit that is electrically connected to the        first resonator or other resonator for foreign substance        detection and can oscillate at a first frequency f1 which is        lower than a resonance frequency fr of the power reception        resonator and at a second frequency f2 which is higher than the        resonance frequency fr;    -   a measurement circuit that is configured to measure a physical        quantity which varies depending on input impedance of the first        resonator electrically connected to the oscillation circuit or        the other resonator; and    -   a judgment circuit that is configured to judge on presence or        absence of a foreign substance in the vicinity of a power        transmission resonator, based on a change in the physical        quantity measured by the measurement circuit when the        oscillation circuit oscillates at the first frequency f1 and a        change in the physical quantity measured by the measurement        circuit when the oscillation circuit oscillates at the second        frequency f2.        [Item 20]

The wireless power transmission system of item 19, wherein

-   -   the multiple steps include a first step of measuring the        physical quantity by the measurement circuit when the        oscillation circuit oscillates at the first frequency f1 and a        second step of measuring the physical quantity by the        measurement circuit when the oscillation circuit oscillates at        the second frequency f2, and    -   the measurement circuit is configured to perform the first and        second steps before and after one power transmission process.        [Item 21]

The wireless power transmission system of one of items 16 to 20, wherein

-   -   the multiple steps include two steps of measuring a same        physical quantity and a step of judging on presence or absence        of a foreign substance through calculation based on the physical        quantity measured in the two steps, and    -   the foreign substance detection circuit is configured to perform        each of the two steps before and after the one power        transmission process.        [Item 22]

The wireless power transmission system of one of items 16 to 21, whereinthe foreign substance detection circuit is configured to changefrequency of performing each step depending on time taken for each ofthe multiple steps.

[Item 23]

The wireless power transmission system of one of items 16 to 21, wherein

-   -   the power reception circuit has a transmission circuit that is        configured to transmit a communication packet for feedback        notifying output variations in the power reception circuit,    -   the power transmission circuit has a reception circuit that is        configured to receive the communication packet, and    -   the control circuit is configured to control the foreign        substance detection circuit such that a process using the        foreign substance detection circuit is not performed in a        session in which the reception circuit is receiving the        communication packet.        [Item 24]

The wireless power transmission system of one of items 16 to 23, whereinthe foreign substance detection circuit is configured to perform twodifferent measurement processes before and after the one powertransmission process and perform a process to judge on presence orabsence of a foreign substance through calculation based on results ofthe two measurement processes.

[Item 25]

The wireless power transmission system of item 24, wherein the foreignsubstance detection circuit is configured to perform the calculationbased on the results of the two measurement processes during a powertransmission session following the two measurement processes.

[Item 26]

The wireless power transmission system of one of items 16 to 25, wherein

-   -   the foreign substance detection circuit is configured to:    -   measure physical quantity that changes as the foreign substance        approaches;    -   judge on presence or absence of the foreign substance by        performing the calculation based on the physical quantity;    -   omit the calculation based on the physical quantity and judge        that the foreign substance is not present when a same        measurement result as the last time is obtained in a foreign        substance sensing process following judgment that the foreign        substance is not present.        [Item 27]

A power transmission device, including:

-   -   a power transmission circuit that is configured to convert        inputted direct current energy into AC energy and output the AC        energy; and    -   a power transmission resonator that is configured to send out        the AC energy outputted from the power transmission circuit,    -   wherein the power transmission circuit has:    -   an inverter circuit that is configured to convert the direct        current energy into the AC energy and output the AC energy;    -   a foreign substance detection circuit that is configured to        detect a foreign substance in the vicinity of the power        transmission resonator by performing a foreign substance        detection process including multiple steps; and    -   a control circuit that controls the inverter circuit and the        foreign substance detection circuit such that the power        transmission process using the inverter circuit and a process        using the foreign substance detection circuit are alternately        repeated, and    -   the foreign substance detection circuit is configured to perform        different steps included in the multiple steps before and after        one power transmission process.        [Item 28]

A program to be executed by a computer mounted in a power transmissiondevice including a power transmission circuit that is configured toconvert inputted direct current energy into AC energy and output the ACenergy and a power transmission resonator that is configured to send outthe AC energy outputted from the power transmission circuit, the programcauses the computer to:

-   -   perform a foreign substance detection process including multiple        steps to detect a foreign substance in the vicinity of the power        transmission resonator;    -   alternately perform a power transmission process using an        inverter circuit and some steps of the multiple steps in the        foreign substance detection process; and    -   perform different steps included in the multiple steps before        and after one power transmission process.

The techniques in the present disclosure can be utilized in a chargingsystem that performs charging of an electronic device such as a smartphone, a tablet terminal, a mobile terminal or in a motor-driven machinesuch as an electric vehicle. According to the embodiments of the presentdisclosure, the risk of abnormal heat generation of a foreign substancewhich is present between a power transmission coil and a power receptioncoil can be avoided.

What is claimed is:
 1. A power transmission device comprising: aninverter that generates first AC power and transmits the first AC powerwirelessly to a first resonator of a power receiving device via a secondresonator; an oscillator that generates second AC power which is smallerthan the first AC power, and transmits the second AC power to the firstresonator via a third resonator; a foreign substance detector circuitthat performs a series of multiple processes to determine whether or nota foreign substance is present between the first resonator and the thirdresonator based on a physical quantity at the third resonator, thephysical quantity varying depending on the second AC power; and powertransmission control circuitry operative to: cause the foreign substancedetector circuit to perform the series of multiple processes before astart of a transmission of the first AC power; cause the inverter tostart the transmission of the first AC power if the foreign substancedetector circuit determines that the foreign substance is not present;repeat a foreign substance detection period and a power transmissionperiod alternately where the foreign substance detection period is aperiod in which the foreign substance detector circuit performs one ofthe series of multiple processes and the power transmission period is aperiod in which the inverter transmits the first AC power, after thestart of the transmission of the first AC power; and cause the foreignsubstance detector circuit to divide the series of multiple processesand determine whether or not the foreign substance is present as aresult of performing all of the divided series of multiple processes,wherein the series of multiple processes includes: two or more types ofmeasurement processes to measure physical quantities at the thirdresonator which vary depending on the second AC power; and adetermination process to determine whether or not the foreign substanceis present, based on a value computed from the physical quantitiesmeasured at the two or more types of measurement processes, and the twoor more types of measurement processes includes a first type ofmeasurement process in which the foreign substance detector circuitmeasures the physical quantity when the oscillator oscillates at a firstfrequency f1 which is lower than a resonance frequency fr of the thirdresonator, and a second type of measurement process in which the foreignsubstance detector circuit measures the physical quantity when theoscillator oscillates at a second frequency f2 which is higher than theresonance frequency fr of the third resonator.
 2. The power transmissiondevice of claim 1, wherein the first resonator has a parallel resonancecircuit including a coil and a capacitor, the physical quantity at thethird resonator is an input inductance of the third resonator, and theforeign substance detector circuit: measures an input inductance valueLin(f1) of the third resonator when the oscillator oscillates at thefirst frequency f1 in the first type of measurement process and an inputinductance value Lin(f2) of the third resonator when the oscillatoroscillates at the second frequency f2 in the second type of measurementprocess, and calculates a coupling coefficient k in accordance with anexpression of k²=1−Lin(f2)/Lin(f1) to determine based on the computedcoupling coefficient k whether or not a foreign substance is present, inthe determination process.
 3. The power transmission device of claim 1,wherein the first resonator has a parallel resonance circuit including acoil and a capacitor, the physical quantity at the third resonator is aninput inductance of the third resonator; and the foreign substancedetector circuit: measures an input inductance value Lin(f1) of thethird resonator when the oscillator oscillates at the first frequency f1in the first type of measurement process and an input inductance valueLin(f2) of the third resonator when the oscillator oscillates at thesecond frequency f2 in the second type of measurement process, andcalculates a ratio of the Lin(f1) to the Lin(f2) to determine based onthe calculated ratio whether or not the foreign substance is present, inthe determination process.
 4. The power transmission device of claim 1,wherein the first resonator has a parallel resonance circuit including acoil and a capacitor, the oscillator is a self-exciting circuit, thephysical quantity at the third resonator is a frequency of a voltageapplied to the third resonator, a square of an oscillatory frequency ofthe oscillator is inversely proportional to an input inductance value ofthe third resonator, and the foreign substance detector circuit:measures a frequency f1 of the third resonator when the oscillatoroscillates in the first type of measurement process and measures afrequency f2 of the third resonator when the oscillator oscillates inthe second type of measurement process, and calculates a couplingcoefficient k in accordance with an expression of k²=1−f1 ²/f2 ² todetermine based on the calculated coupling coefficient k whether or notthe foreign substance is present, in the determination process.
 5. Apower transmission device comprising: an inverter that generates firstAC power and transmits the first AC power wirelessly to a firstresonator of a power receiving device via a second resonator; anoscillator that generates second AC power which is smaller than thefirst AC power, and transmits the second AC power to the first resonatorvia a third resonator; a foreign substance detector circuit thatperforms a series of multiple processes to determine whether or not aforeign substance is present between the first resonator and the thirdresonator based on a physical quantity at the third resonator, thephysical quantity varying depending on the second AC power; and powertransmission control circuitry operative to: cause the foreign substancedetector circuit to perform the series of multiple processes before astart of a transmission of the first AC power; cause the inverter tostart the transmission of the first AC power if the foreign substancedetector circuit determines that the foreign substance is not present;repeat a foreign substance detection period and a power transmissionperiod alternately where the foreign substance detection period is aperiod in which the foreign substance detector circuit performs one ofthe series of multiple processes and the power transmission period is aperiod in which the inverter transmits the first AC power, after thestart of the transmission of the first AC power; and cause the foreignsubstance detector circuit to divide the series of multiple processesand determine whether or not the foreign substance is present as aresult of performing all of the divided series of multiple processes,wherein the series of multiple processes includes a first determinationprocess to measure a first physical quantity corresponding to a firsttype of foreign substance and determine whether or not the first type offoreign substance is present, and a second determination process todetermine a second physical quantity corresponding to a second type offoreign substance and determine whether or not the second type offoreign substance is present, and the power transmission controlcircuitry causes the inverter to transmit the first AC power between thefirst determination process and the second determination process.